If he's only on one solids feed a day he could still have a couple of days between poos which is common for breastfed bubs. If the poo is soft when he does it and not hard and dry then he's probably not constipated.

What are you feeding him? Rice cereal can bind then up, so can banana.
Fruit such as apple or pear can help with constipation if you think it may be constipation.