It depends on how long you want it to last. I would suggest going to a mattress store, not a furniture store since they work on commission and would suggest the higher end. Let them know how long you want the mattress to last and that it is for a small child, they can make recommendations. Also, what kind of bed do you want--something he can use for a long time or just for a few years. Consider something that could transition to 5 or 6 yrs old or even 10. If $$ is a big factor, you could check out IKEA for a bedframe. They are reasonably priced.