***Not a teacher***
...he would have been able to come (if he had wanted to)If he hadn't been busy, he could have come to the party.
I know for a fact that he would have definitely been there. It was only his being busy that stopped him.If he hadn't been busy, he would have come to the party.
Being busy stopped him from coming, but even if he had not been busy, there may have been some other reason for him not to come (he didn't want to, he would have gone out with other friends etc). i.e. it is a possibility that he would have come, but not definite.If he hadn't been busy, he might have come to the party.