My son was a supported sitter at 5 months, he hadn't quite gotten the hang of it yet. I got him a little bumbo chair that had a tray and toys around it that seemed to help because he had to stay sitting up in order to play with the toys. I would also set him on the floor with the boppy pillow behind him too

The best way to train him to sit up is tummy time. Every day for at least 15 minutes. His core muscles have to develop.

All babies develop at their own pace. If you're concerned, talk to the pediatrician.