Hello. Well, a nice thing to do, in my opinion, would be a little investment in the old bike. Something like pimp my bike thing. Nothing excessive , maybe a paint change in some colors he prefers or with some symbols that he likes. Depending on his age and things he like, you could just get him some cool gadget for a bike, if trere are some on the market. If you put some effort and imagination and make it a surprise he will appreciate it, be sure of that. I really can not think of something cool for kids nowdays, except the paint with some symbols of his favourite sport club ( football, baseball,etc.) It may be a challange for you but if you get it right, a smile on your boy's face will be worth it. And of course make sure that the bike is in a top shape, tyres and everything.