I suppose you know you are straight and you assume your friend is straight too. If so, he wants to remain friends forever just as he said.
If, however, you suspect he's gay and testing the water with you, you could deflect it by saying that 'I'm sure when we both have wives and kids, we'll still be friends' - or you could ask him outright and say 'Wow, you haven't got a crush on me have you?' That'll bring him up short. He'll have to laugh and deny it or come clean and tell you he loves you. So, think about how you are going to handle that before you ask the question. How are you going to let him down gently (if you like him that is). You just have to be nice but firm. 'Sorry, you are a fantastic best friend forever - but I couldn't fancy you poached because you aren't a girl.'