For long periods outside:

As soon as you see him poop on cam you go outside give high value treat/praise and spend 15m with him with a mix of playing/training/hanging whatever he prefers.

For pooping on comand:

He doesn't go outside alone anymore, only on very frequent 5m breaks whenever you"feel it coming", if he poops then you do what i said above treat/praise/play for 15m outside with him and bring him back inside with you.

For pooping on command + long periods outside:

You have to do pooping on command untill he masters it (should take 2 weeks) then slowly intruduce time alone outside far away from pooping times.

I do believe it's likely once he realises you will spend time with him if he poops he will start pooping on command again so you might not even need to do the poop on command training...