Here is how you tell a man you like having sex on top....
When in bed with him say "You know? I really like having sex on top. Let's do that more often."
You say you don't want him to "feel insecure"?
It doesn't sound like you mind feeling insecure around him.
Good sex is always always always about good communication.
If a person is comfortable enough to have sex with someone, they are in the catagory of being comfortable enough to tell each other what they want.
He's no more experienced than you, but it seems you are ok with him letting you know what he wants, but you don't feel you can.
Chances are, he would like to know what you like.
When sex is better for you, it's going to be better for him.
If you don't like a position, especially because it's causing you discomfort, you are in no obligation to do it. In fact, you have the responsibility to tell him when something hurts, or you don't like it.
The days of "He's the man, the sex is for him" is long past my dear.
You have every right to enjoyable sex as anyone else.
I would like you to get this book. It's called "Vagina" by Naomi Wolf. In addition, I would like you to actually read it.
It is written by a woman, and for women. But, can be very helpful to men too.
At one point in my long marriage problems started to occur in our sex. Not through lack of affection, but because of physical factors.
I would lie in bed next to him reading from that book. It was a very intimate thing, and he was facinated. It helped a lot.
You may not be comfortable reading to him.....but you need to read the book for yourself, to understand your own mind and body better.
Finally, if a man is going to feel insecure because another person lets him know what he can do to please them, he's not much of a man.