M.S.
It sounds totally normal. Breast milk is almost all good stuff, so very little of it needs to be pooped out, hence the once a week poop. Formula on the other hand is more like adult food... Lots of waste in it. Babies that are formula fed will go much more often because there is so much more waste. Normally, formula would also cause it to be firmer or thicker, but since you are feeding both, pretty much anything in that spectrum would be considered normal.
If you are worried about allergies, be on the lookout for tummy pain, blood in his stool, vomiting, or rash.