Adults don't eat mash and they poo OK
It's not unusual for BF bubs to only poo once a week. His little body is still getting used to processing solid foods, same as any bub fed any way. There is also a stage when they really become aware of the sensation of doing a poo and often don't like it.

If you want to try an alternative how about trying more high fibre foods that you offer? Pear is a good one. Avoid the binding type foods.