P.G.
What Marda P said. More details will help get better answers. I have a 7 year old on the spectrum, so hopefully I can give you some insight. He is mainstreamed in a regular class, is a sensory seeker (needs a lot of sensory input) and moves around a LOT - his body needs that.
I highly recommend a blog by a mom of 2 kids, 1 on the spectrum, 1 not - she's been writing for the last 8 years, since her daughter's diagnosis, and is a WEALTH of information and supportive attitude. Her readers are the same. Many are either parents of kids on the spectrum, or are autistic themselves. THAT may be the best resource you can find - autistic adults. They are blogging their experience and can give you a window into the way your son thinks, reacts, etc.
I have a facebook page - autistikids - that has many links to helpful pages, blogs, etc., of counselors, autistic adults, parents of autistic children, so feel free to visit and look around. You might find some very helpful resources there. Also visit AutismSpectrumExplained.com - written by sisters - one autistic/on "mostly typical" :)
One thing you need to realize, is that he is not going to have the same social behaviors as his siblings. His brain is wired differently. If you can find his strengths, the things he loves, the things that comfort him, that will help you a lot. Also, what makes him uncomfortable? Is he sensitive to touch or does he need to move? If you/ he can learn his sensory needs, that will make things easier to deal with as well.
Also, be aware that siblings get on each other's nerves, and they could be pushing his buttons on purpose, which isn't good. Be aware of that and stop it if it happens - for all of them. He needs to learn that it's not ok to do things that annoy people also.
People on the spectrum can be VERY sensitive to the emotions of others, so if you are stressing, he can probably pick up on it and it will make HIM more stressed (my son is like this). By keeping my calm, it helped him too.
He breaks rules possibly because 1) he doesn't really understand them, 2) he hasn't internalized game playing the way typical kids do (this is VERY common - school should help with this if he has any OT/social support at school), 3) no one's really explained them to him and have just expected him to "get it". This is huge - people on the spectrum generally don't just automatically "get it". Things we take for granted have to be taught/learned - like turn taking, game playing - stuff that typical kids learn a lot earlier and somewhat more "automatically".
Re. playdates - if he has 1 friend that gets him, take advantage of that. If he has any autistic friends, foster that relationship. Don't force it because it's the "thing to do" and its what you're other kids did. He's not on the same wavelength as they are. You'll have to find out where he is and go from there - that's ok.
He needs a safe place to go to "escape" to when things are too much. Sensory overload is very common for autistic people and the need to be alone to decompress is very important. Find out what comforts him and make sure he has access to that when he needs it. And his siblings HAVE TO respect this. Heck, they should all probably get a "quiet place" for themselves.
Feel free to IM me if you like, and you can email me at ____@____.com!