Hey Dee cyrus did not start sitting up till 6 months and started crawling about a week later. Try putting him on his stomach and he should start trying to move this will help strengthen the lower muscles in the back and adominals.
Givo sat at 4 months unsupported. I never used the boppy pillow. I just would sit him on the floor and make him grab his toes and within a week he could sit unsupported and without grabbing his toes. It really helped me out when he could sit because then he could grab the toys around him and keep himsaelf entertained.