I understand where you are coming from, you wanted the party to go perfect for your son, who wouldn't it's a very special day! But as you've found out, things don't always go that way, and it's easy for someone to tell you what to say or do when they weren't in your very situation. Everyone "wants" to say this or that to the kid or the parents but in that peticular situation, I know it's easier said then done. You want to be the nice fun host, not someone going around putting kids in time outs or having to get the parents attention to watch their own kid, whom they should have been watching from the beginning.
I've been in your shoes and it's funny because I don't ever remember kids behaving this way when I was a kid. I remember birthday guests sitting around watching me open presents and keeping their hands to themselves so maybe you remember your parties being this way and haven't learned how to handle to "out of control" ones at your own childs party just yet. I know it's frusturating, you WANT more than anything for the PARENTS to speak up but that of course doesn't always happen, Nattalie O. had a good point, if the parents KNEW their child was misbehaving, more than likely they would have done something already and since they didn't it's better to keep your cool and try your best to enjoy the moment and remember not every party will be this way.
I've caught my own daughter grabbing her cousins presents and trying to take off with them BUT I quickly caught her and told her "no, no that's your cousins new toy and we should let him decide if you can play with it". There's no need for party guests to think they have rights to all of the gifts, that's what party favors/goody bags are for! At least that was always my favorite part about going to b-day parties, I never thought for one second I had the right to another ones gifts. One thing I've learned that worked over the years was to have hubby quickly put the gifts away and get the kids on to the next exciting event of the party. If you have the party outside of the home, bring a large trash bag and bag up the gifts right after opening and have hubby bring them out to the car.
It's much easier having some sort of plan ahead of time so you're not having to stress over out of control kids or irresponsible parents and keep in mind, not every party is going to go perfect but every year you will have another chance to have a great party and enjoy every moment of it that you can, they grow up so fast! Take care and hang in there, hope you enjoyed what you could have!