1st Mattress for my 5 year old

As the title says, I’m looking for a mattress for my 5 year old son. This will be his first “big boy bed” so I want it to be comfortable for him. I’ve only ever bought 2-3 mattresses and most have turned out to be duds. So I thought I’d ask for advice.

Need a full size mattress for him, kinda on the firm side and not memory foam as he runs hot. Well more than likely be spending nights in his room too so want to take that into consideration.

My husband and I have the purple mattress and love it, but it’s quite pricy to be buying for a 5 year old I feel. My budget is around $500-$700 max.

Any recommendations are helpful! TIA!