Whats the best way to slowly help my chunk sit up? We cant use the couch because he is constantly…

Whats the best way to slowly help my chunk sit up? We cant use the couch because he is constantly spitting up, and it goes far. I don't have a bumbo, or a sit me up seat. He already stands while holding my hands, and lifts himself up on his belly. I need help trying to figure out what to do to help him sit. When I try he plops to one side.