Oh I think karma will oblige.
Not least because someone who runs out on his sick child one month and is engaged the next must be, at some level, the epitome of selfishness.
So presumably that selfishness and attendant cruel streak will at some point ruin his new life/relationship too. I would be astonished at his girlfriend's naivety, but presumably if they got engaged so quickly after your split they were seeing one another prior to it, so it's quite likely that they're equally self-obsessed and selfish. They deserve each other.
The likelihood is that by the time he starts treating his new relationship with the contempt he's shown elsewhere, you will have moved on and be enjoying life, and will end up pitying them both.
You are so, so lucky not to have ended up spending the rest of your life with a weasel like this.
While you and your DS were faced with this dreadful news and the prospect of a long difficult time ahead, he was getting his head turned like a loved-up teenager. And that took priority for him.
Any father worthy of the title would have had all thoughts of new relationships, getting engaged, etc. fly out of his head the moment you heard the news, new life plans instantly put on hold. He is beyond disgusting.
Are his other children much older? Presumably due to your ds's age he would have to liaise with you to have him at the wedding... he may be telling himself different, but it will be partly SHAME that will have kept him from doing that, and kept the wedding from you and DS.
Pity him. His biggest punishment is having to live with being the person he is. Ugh.
Oh, and I can guarantee you that not only will everyone that knows the situation be disgusted at him, even if privately, they will also have scant respect for the seriousness of a wedding which happens right in the wake of such a mess. He's making a fool of himself, basically, even more so she. Wow, 'engaged' eight weeks after walking out on his sick child, and married just a few months after that! They must be soulmates.
Good luck, you will get stronger for this, you will move on, you will look back and be relieved and delighted one day that your happiness has nothing to do with him.
I hope your DS's treatment is going well and that he will soon be on the road to recovery.