I think it's a bit complex. Obviously he was trained when he wasn't in your care. It seems the last caregiver had a different method.

I usually do observations to figure out things.

1. Is he independent in the washroom or needs assistance.
2. Is he successfull in a potty or able to use the toiled and can he reach it or needs help.
3. Type of clothing
4. Diet
5. Can he go on his own or needs to tell? Maybe he is just lazy to tell. I have independent kids so they go on their own even toddlers. Each has their own potty (so no fights or misunderstandings there)
6. Do you carry a potty when you go outdoors.

The more friendly you do it, will reinforce a behaviour of doing it where it should be.

During my 9bsrvations I figure out that actually they do behave differently at daycare and home. Ask them in details. How he used to go to the bathroom clothing, with assistance or not etc. Reminders etc.

That could help you figure out what is going with this kid.

Good luck