I really think you need to stop the incidental contacts and communications. For that you need fixed and regular handover arrangements in place.
Start making adjustments to how you communicate. For example instead of asking what time his bus gets him home, tell him you will be dropping them off at x time. This should be a reasonably agreed and regular time that allows him to get the bus home. If he is not going to be there it will be up to him to let you know. You either then fit in with his plans or tell him you can’t make it.
Meet at the PT night outside the school. Tell him the dates and times but leave it up to him to be there. If he wants to be there. He shouldn’t need a girlfriend to tell him he should go.
He doesn’t seem to have boundaries so you need to put them in place. By the sounds of it he doesn’t have a car but that is his problem. Don’t facilitate him by offering lifts. You don’t have that relationship anymore and you can’t cope with his behaviour when you meet up.
Keep a record of his failures to comply with the agreed arrangements, it will come in handy. At some point you will probably need to formalise with a court order.
Look he has a new girlfriend and that makes you feel bad. You will get over it because he is no prize. He is a man who can’t prioritise his own children. Who doesn’t have reasonable boundaries and is jumping into a new relationship too soon. He will probably get dumped or she will have to accept his fuckwittage. I mean they have been together weeks and she is already looking after him, running him around and sorting out his parental responsibilities. He’s a loser and she’s a mug. He’s a user and she is being used.
Get own with your own life including repairing the damage done by this relationship and split. You are facing up to it, he isn’t and he will always be a useless man child. Photoshop his head into a toddler body and remind yourself that this what you have lost and what she has gained. Look at it everytime he does one of his fuckups, not being home on time, not wanting to go to PT meeting, etc.