That's tough. I'm also a carer for my ASD son, so I have some idea of what's going on. If I were in your partner's shoes, I'd take my son to a child psychologist (who specialises in ASD) to help my son work through it. Thing is, as much as his son is struggling with his dad's new life, his dad shouldn't let his son limit who he sees or dates. If his son won't see you, there should also be the option of having someone else care for him while you and your new fellow go on dates.
I think this can be worked around if your partner is willing to work with his son. However, if he's not willing to put in place strategies to make this work, then I think you'd be wise to walk away.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.