My firstborn would poop 2-3 times a day when he was young; my younger son would go about once a week. *But* it didn't seem to bother him. (And he still only goes about every 2-5 days.) Every baby is different!
It is extremely unusual for an exclusively breastfed baby to be constipated, even if they don't poop every day, but only every week. Breastmilk is usually considered laxative, and sometimes the baby absorbs the breastmilk so perfectly that there isn't much waste to poop out.
Check with your pediatrician before giving him anything by mouth, though. Giving a breastfed baby water can mess up his system (weird, huh?), so make sure that it's okay in your baby's case.
I'd do daily belly massages, and "bicycling" his legs, and other things to encourage him to go. Check his belly for firmness, and if he seems to be uncomfortable, you may want to do another suppository now. I don't think he'll get accustomed to suppositories (though you may want to check with your doctor on that), and it's possible that his bowel became used to not pooping, so he may need a few suppositories regularly to get him regular again, and then you can discontinue it.