Struggling to poo at 18 months old :(

Try the prune pouches for 6 month olds they’re a god send and we use them every 2 to 3 days to help my son poo (he turned 18 months today)

Before switching milk, I would increase fibre in his diet, ripe fruit such as banana, apples maybe fruit pots/pouches and plenty of vegetables and water .. reduce white bread, rice, pasta and go with wholemeal options.
If that doesn’t work I would then look at what else it could be.

Knees higher than hips, like squating when the stool is hard, muscle not strain much and easy to flush out... give more water, sometimes hot water or warm water works, riped bananas before bed time do work... if already on cow's milk you can dilute the milk and can give him... check baby gut health foods it could help you...

For my little girl I switched from whole milk to semi skimmed and that helped with this. It wasn’t as bad as what you’re describing but similar.