6 month not pooping!

My baby is almost 6 months and the last few weeks he hasn’t been pooping regularly. The last time was a whole week in between and now he hasn’t pooped in 9 days! He is a breastfed baby. But I have recently given him formula every once in a while when I can’t breast feed him and we started him on some solids. I have tried everything to get him to poop. Prune juice, sweet potatoes, pears, apples, warm bath, baby massage. Do you think it’s because I gave him formula that this all started? Or normal once starting Solids? He is passing stinky gas normally and doesn’t seem uncomfortable but I am getting worried...