Any stores/parks that he really likes going to? Maybe take him there and prepare yourself to march straight back to the car and head home if he doesn’t want to wear the mask. Make it very clear that you will only stay for however long he keeps the mask on. It may take a few times of actually heading home but his desire to go may overcome his hatred of the mask where he’ll finally keep it on. My daughter (almost 3.5 yrs) fortunately had zero issues with masks and actually seems to enjoy wearing them. That being said, there has been a few times where she gets into a snit and decides to take her mask off in protest, at which point I say “ok fine, if you don’t want to wear the mask then we have to head home” and turn to head home. She quickly yells “No!” and puts it right back on.