A. You're right that you don't want to use a suppository to 'help' your baby have a bowel movement each time.
Since this started at about the time you started supplementing breastfeeding with formula, one simple solution might be to simply stop giving formula and just breastfeed. Exclusively breastfed babies rarely become constipated.
If not supplementing isn't an option, you might try changing the type of formula you are using to supplement him with. For example, if you are using a regular milk based formula, like Enfamil Lipil or Similac Advance, then you might change to a soy based formula, such as Prosobee or Isomil.
Lastly, you should consider that some breastfed babies begin to have infrequent bowel movements at this age. If not having regular bowel movements doesn't seem to be bothering him and they are soft or watery when he finally has one on his own, than it might be normal that he is going 5 or more days with no BM.
A recheck with your Pediatrician, to make sure he is gaining weight well and had been having normal bowel movements in the past, would also be a good idea.
See our guide to breastfeeding and constipation for more information.