Homeless guys need love too. So go for it. But I would put down a few rules that he would need to adhere to.
1. No overnight stays at your place. This will reduce the chances of his moving into your home just for a place to stay.
2. All intimate activity only occurs at his place, whether that be a car or a box in an alley. This also helps assure he will not move in with you.
3. When the two of you get together, he must shower first. This can be either at a local homeless shelter or using a hose in your yard. But having a clean date will keep you from being overly embarrassed by the accumulation of weeks of body odor.
4. For the first few dates, each of you pay your own way. This will avoid that awkward moment that happens when the check comes and he looks at you with those innocent eyes and reminds you he is homeless and hands you the dinner check. This will also lessen the odds of him simply seeing you to mooch off of you.
5. After the first few dates, you make it clear that if he wishes to continue dating you, that he will be responsible for paying for the evening. This will motivate him to find a job (if he does not have one currently) or set aside money each week from his job in order to see you. His agreement (or lack of agreement) will point to how he values you.
6. Last but not least, do not offer to move in with him. Cars and boxes can be difficult to live in since they provide very little privacy. Once he has been successful in getting off the streets, you can talk about taking the relationship to that level. Again, what effort he gives to accomplish this will show how much he values you.
While I wrote this in a manner to provide a small chuckle, there is a grain of wisdom in a few of these points. Good luck.