Aggression is an occasional problem with my autistic 5-year old as well. When it first started at age 2 1/2, it was more than frustrating -- it was terrifying. I always said I would never consider an institution for him, and I stand by that, but I also have two other children to keep safe. Controlling him at age 2 was difficult enough -- the image of him exhibiting the same behavior at age 15 when he was bigger and stronger than me made me physically ill. We have a pretty good handle on the situation now, and here's what worked for us:
1) Keeping a functional behavioral analysis -- basically, a record of each incident with details like when/where/who, what happened, how long it lasted, what was going on right before, how it ended, etc. I have a form, if you're interested, but really, it just lists the things I said with spaces to keep track of incidents. This can be helpful to locate patterns and triggers and reinforcers. If you can find them, it may help, but not always -- in our latest round of aggression, my son was being reinforced by his sister's extreme reactions, and a 3-year old with special needs can't easily tone down her reactions.
2) Rigid scheduling. Most kids on the spectrum do well with routines, and the key for my son was scheduling every minute of his day so he didn't have "free time" to go off on an aggressive tangent. Picture schedules were crucial for this because they let him know what was happening and what was coming so he always knew where his mind should be. This is not easy -- it's extremely wearing on the whole family. Unsupervised time was unfocused time when he could go off. However, it wasn't permanent. After a few weeks, after the aggression habit was broken, we could experiment with allowing free time. It worked. Now, usually when we're having an aggression cycle, a few days of rigid scheduling breaks it.
3) Social stories. There are books about this (Carol Gray? is the author), but basically, they are very simple stories with simple pictures that demonstrate appropriate behavior. They work.
One thing to keep in mind: many kids on the spectrum have recurring "themes." That is, the behavior goes away, but it comes back again, maybe in a different form. Aggression is one of my son's themes. We know not to get rid of our picture schedules. We may not use them for months at a time, but we will inevitably need them again. This is why it's so important to establish an action plan to keep in the back of your mind, even after you have the current situation under control.
One other thing: you may necessarily have to restrain your son to protect your 10-month old. Be careful not to use more force than necessary. And don't use corporal punishment. Gentle Discipline philosophies aside, there are practical considerations for dealing with an autistic child. Most autistic children do not read social signs well. This means that they learn what they are taught, without consideration for appropriateness. So if you teach them hitting, they will hit. Period.
If I think of anything else, I'll post it. Feel free to ask questions, here or in PM.
All the best,