Agree with all the advice given. Take care of yourself. He is taking advantage of your goodness in the worst possible way. He should take care of dad when he is not at work.
Sorry to say, you are enabling his affair by taking care of dad while he gets to spend time with his XGF who is now his GF again.
And agree don’t try and talk him out of this, just take action.
Gordie 40s W 40s M20+ kids 2016 BD W fantasy affair w OM1 I do everything wrong 2017 I start to DB W says TLTL files for D PA w OM2 2018 I do LRT W drops filing and OM2 situation slowly improving