That is exactly what I think. It's a comfort thing, although initially I thought it could possibly be using it to hide his belly a bit? Maybe he is a bit self conscious about his body? But when he props his beer on it, I think it's just his pseudo tray table.I reckon the cushion is just to make him more comfortable and give him a ledge to rest his arm and beer on. It might not necessarily be his preferred position/place to sit in either and just does it when they're filming. I find the sweat more disturbing.
*and the sweat bit, too.