It is hard we had a Snap n Go too good capsule but DS got hot too. Pointing the air con at the roof and having it on quite hard seemed to help. Or if you have vents in the back direct them at the back of the capsule. We still ended up with a sweaty back though.
Be careful with a liner, make sure it doesn't interfere at all with the harness and is very thin, nothing bulky.

There is a liner you can get to keep the seat/capsule cool. but for when they're not in the seat though. No good for when they're in it. But it could give you a little extra time before he gets hot if the seat is extra cool when he gets in I guess. I'll try to remember what they're called.