I would be going bottom free for a week and staying home as much as possible. Pack up good rugs and pull out old towels for him to sit on etc. I think you should break the habit of him going in his pants and encourage the toilet.

When he has an accident I would just say 'oops' and run and take him to the toilet and say toilet is for wees etc.

I had to explain to my dd that when I felt wee and poo in my tummy and it needed to come out I had to run to the toilet. For some reason that was a lightbulb moment for her and she was easily trained day and night within weeks.

Hth