when our DD was that young, she didn't care for a lot of tummy time. Heck... still doesn't. We used to just give her about 10-20 secs after each diaper change. Really, not a whole lot. Now, she still gets VERY tired for tummy time at 3 mos, so we only do tummy time in the morning, but it lasts a little longer now, like 5 min. - 10 min.
one thing she did like though, is tummy time on one of our tummies. I would hold her so her belly was facing mine, and then leaned all the way onto my back. Sometimes I would hold her feet and let her push and she would "crawl" up to my face. But you still do have to pay attention to make sure he doesn't get too over tired.