I really feel you on this, it can be so draining after a long day when they just want to fight everything. My girl went through a similar phase around that age, and what helped us was letting her bring one small toy into the bath so it felt less like an interruption to her play. Since it seems to mainly happen after school, maybe he just needs a short wind-down at home first before you bring him to the bathroom, so he feels like he had some control over the transition.