I started him out on his back just to get used to the idea of the mat and to have an opportunity to "take it all in". He did well so then I propped him up on his whale pillow and turned him over on his tummy.
He lasted about a minute before he started getting upset. I think one of the things he hates the most is the texture of the mat. It's very slippery for a little guy and he can't get any traction/leverage. The little whale pillow is made of the same fabric so he just slides around. Maybe it won't be such an issue as he gets stronger.
Once he had enough of tummy time, I flipped him back over so he could enjoy the mat on his back but he wasn't a big fan by that point. We put the mat up & called it a day. We'll get it back out in a couple of days and see if we have any better luck!