My DS is almost 8 months old and is very happy to remain sitting. He can roll but only does it on the bed or couch, never on the floor. It is actually really handy because he isn't really mobile. He can butt scoot if he really wants to but doesn't very often. If he does want to go somewhere he waves his hands so he can hold someones hand and walk wherever he wants to go. I can't even do tummy time anymore because he goes onto his hands and knees and then back to sitting lol.

I don't think he will be a blob at 21, he is just taking his time. I bet you he can reach things if he thinks you aren't watching lol