shanhun, I can understand how you feel about this relationship and why you are wondering whether it has a lasting future.
But I don't think you are, at all, wasting your time with this man, because you like being with him, you say you love him, and you can even imagine spending the rest of your life with him. As long as the relationship has those positive aspects, and is satisfying in the present, just enjoy being with him. None of us knows how a particular relationship will turn out in the future, and this one doesn't sound particularly risky, or a bad bet.
It's good that this man loved his wife, and that his memories of her, and his marriage, are good ones. Not only does that suggest that he's not saddled by a lot of guilt and remorse and regret and unresolved conflict regarding his wife and marriage, it also suggests that his grieving process may be considerably less complicated and lengthy than it might be if that were not the case. This man really liked being married--which is going to make him want to re-marry probably sooner rather than later. And, right now, he is thinking of you in that regard.
He may simply need more time to fully dissolve the bonds of his first marriage in his own mind and heart. He needs to keep his happy memories of his wife and marriage, but he does need to displace his commitment and current feeling of attachment from her to you. He does need to begin taking her clothes and shoes from the closet, and storing them or giving them away, because being able to do that, as painful as it is to do, helps in the grieving process because it is a recognition of his changed reality, a recognition that her physical presence in his life--and his bedroom--is over. It is further recognition that his marriage is over, and it's that recognition that will help him to consider another marriage without psychologically feeling like he is cheating on his wife.
It does sound a little like the bedroom has turned into a sort of shrine to his wife--with all the photos, her clothing, and even her ashes. That can't possibly help you to feel comfortable in that room because you're surrounded by reminders of her and so is he. Some of those photos of her should be replaced by photos of you and by photos of you and this man together. Space in the closet should be available for you to use if you stay over often, or if you want to begin living with him. He doesn't have to move her out of his mind and heart, but he literally has to allow you the space to move in, if he plans on continuing a life with you, and that's going to involve cutting down on the size of the shrine.
I think you have to talk about these things with him, simply in terms of how you feel and without pressuring him too much. If you need him to make some changes in that bedroom, so you don't constantly feel like there is a threesome in there, let him know that. You're not asking him to get rid of her, or her place or importance in his past and in his memories, you are just asking him to make room for you in his current life, and that's not an unreasonable request given the basically good relationship the two of you have. This new relationship needs room to grow--and you literally need room in that bedroom for it to happen. So, I think you have to raise the subject of helping him to pack her clothes away, and perhaps putting away some of her photos, or placing them in an album, and replacing them with photos of the two of you, perhaps on one of the vacations you took together. Those photos are part of the history the two of you are building as a couple, and they are something you both can relate to.
The suggestion another poster made about repainting the bedroom and doing a bit of redecorating is not a bad idea. It would be a project you could both work on to make the bedroom a special place for both of you. You could shop for new bedding and window treatments, talk about the kinds of colors and patterns you like, and make it a room you both feel good in. If he is willing to do those sorts of things, it would be another indication of how willing and able and ready he is to move into a new chapter in his life. If the two of you are eventually able to move into a new place, a place that does not contain so many memories of his wife, and a place that would be "ours", that might be even better, for both of you. But first I'd start with just making your presence felt in that bedroom and trying to tone down her presence somewhat. Take it one step at a time.
As long as this relationship is good for you in the present, and you see its future potential, I think you should hang in there. You are sensitive to the fact that he is still mourning a great loss, but his relationship with you is also helping him to deal with that loss by bringing something new, and hopefully wonderful, into his life. So, while a certain amount of patience might be needed in this situation, I don't think that should stop you from expressing your own needs or trying to get those met. He needs to understand your situation just as much as you need to understand his--that's how you'll build a firm foundation together.
People often tend to remember beloved spouses as more perfect than they were, and there is no harm in that. Be happy for him that his memories are such good ones--and let him know that. Be happy for him that he had love in his life before, and let him know you want to make sure that he feels loved by you as well. His deceased wife is not in competition with you, she helped to let him know how good marriage can be for him, and that's why he's talking of marrying again. She took care of him in the past, and helped to make him the man you now love. She's more your friend than your rival. Think about that.