Babies development is affected by size, so if he's small then he may take a little longer with his gross motor skills. My DS was small with short legs, so he took longer to roll, sit, crawl and walk as he had to wait to be big enough / have long enough legs to do these things
I would concentrate on rolling for now, as sitting and standing come much later! I have read that bumbo seats are not ideal as they hold the baby up so he doesn't learn how to use his muscles to hold himself upright. At that age DS was in a sloping chair, or on his back or his tummy. It's also not a good idea to make them bear weight on their legs at this age either, so no standing, just support him full body on your legs.
At that age we'd do rolling practice on the foot end of the big L shaped sofa. I'd roll him so far, then let him do the rest
He loved it! Once he mastered rolling he fairly quickly progressed to sitting. Please don't put him in the highchair until he can sit, as it won't be comfortable for him.
At that age my DS also liked me throwing his soft blocks gently at him
He was an early thrower so this was a good game for him! We also did peekaboo with a cloth, and hiding an object under a cloth. For sensory play he had a chew toy with different textures which he loved, and teething rings to bite and suck on.
He also liked having admiring family and friends hold him and talk to him, and to just watch what was happening in the world
It sounds like you are doing a great job of entertaining him, and he has plenty to catch his interest, so he'll develop his skills when he's ready