Did you mean 16 year old? Because I would say Super lovers or fall in love with me.
Ps: if it's not a typo and you really meant 6 year old I would say he's too young for yaoi or any book or movie that speak about sex (gay or hetero). But you could show him some shounen ai (gay romance but no sex) if you really wanted to show him gay romance
You really are Yaoiismylife and seems forcibly want to make the kid's life like yours. so unfortunate.
NO. use this instead it's very cute http://www.mangago.me/read-manga/nukoduke/
What should I show my 6 year old cousin for his first yaoi(≧∀≦)