@Hannah he’s been fine since on solids but changing over to cows milk I think has made him constipated
Cows milk is high in lactose. He might be struggling with that as it can be hard to digest, and it giving him gas which is making it painful to poo, hence not going. You could go back to his formula, or part formula part cows milk whilst you wait to see doctor? Could try probiotics - they wouldn’t hurt and contain lactase which would help if it’s this. Or indeed just keep him on the cows milk and give him the lactulose as needed.
We’ve had constipation when starting solids which is ongoing. We find prunes, pears and apples help, as well as increasing water intake and lactulose as and when needed.