DS is pretty much the same, he hates tummy time because of his reflux and hasn't yet worked out how to consistently roll himself over yet so just lies there and screams! He likes it if i prop him up on DD's swim ring though, and its low enough that he can roll himself over off it. Another thing he loves playing on is one of those round blow up roller things, i put him on his tummy and roll him forward and back, and also do this with him on his back. Also, something i remember from DD doing gymbaroo (poor 2nd child never gets to do anything fun! LOL), was to do lots of different floor exercises like bicycle legs, arms round and round, crossing opposite foot to hand (this was supposed to be really good for crawling).

They absolutely will do it when they want to, DD has bad hip dysplasia so when she was around this age she went into a spica cast for 12 weeks (thick plaster which goes from under the armpits to ankles with her legs in a froggy position), even with the weight of the cast and being unable to move anything other than her arms she still managed to work out how to crawl in it!