I think if he is excited about it, then go for it!
When we adopted our boys name was Maximas and we legally shortened it to Max at his request (he was 6 at the time) and we let him help pick his middle name and he chose my husbands name. He actually told us that was what he wanted before he ever officially moved in. Every now and then he will ask about Maximas, but normally just to make sure that isn't his name any more.
Funny story - we gave his sister/our dd the same option of picking a middle name and she got really quiet and then jumped up announcing that she wanted her middle name to be Spongebob. I told her that she was too pretty to be a Spongebob, so again she got really quiet and then said, "Fine, I'll be Patrick". Obviously they watched a little too much Spongebob in their foster home

When we told her it couldn't be a name from a cartoon, she asked what my middle name was and decided to go with that.
Good luck! Follow your heart!