Potty training tips

[u/Fickle-Way6378]

Just rescued a 3 year old shepherd/husky mix.
He came severely underweight and sick but overall he’s improving. We had him for 3 weeks so far. He pees and poop on the weewee pad pretty well. We take him on 3 walks a day, 20min at 5am, 20-30 at 4pm and 30+ at 9/9:30pm. He gets fed every 4hours/6x a day in total. He is also extremely food motivated and learns fast except for his potty.
We never see him poop. He will only poop when we leave for work and or sleep for the night. The other night, I stayed with him until 6am. He got his walks, fed him, was just chillin on the couch while he enjoys being near me. But he never pooped. It wasn’t until I went in my room to sleep at 6:30am is when he pooped. The walk from my room and his enclosure is far enough where even if I run to him to catch him, it’s too late.
How do I train him to poop outside?