Hi, I have actually worked with children with Autism before. And my brother has High-Functioning Autism. The behaviors do sound like its a possibility, but only a professional who has studied your child's case can know.
Have you ever heard of PDD-NOS? (Pervasive Developmental Disorder-Not Otherwise Specified). Basically PDD-NOS is on the Autism Spectrum so its practically the same, but slightly different if they can't find out *exactly* if its Autism or not but they think it is. It may be done differently here in American though.
I was a TSS (Therapeutic Support Staff) with a Rehabilitation Degree, but basically my job was being a professional briber lol. So what you want to do is use positive reinforcement (it sounds like you may have started that). The key is to be VERY consistent. And you NEED to make sure you are "pairing" you rewards to your child. For instance, lets say your child plays nicely with your daugter and does not hit her. After a few minutes go up to him and give him something he likes (like a small toy or something like that, i try to avoid candy just bc its not healthy, but if thats what works then all means, try it!). When you hand him the toy MAKE SURE you use verbal praise. So when you hand him the toy, say "You did a GREAT job playing with your sister and using nice hands..." And stress how great a job he did. Pretty much if you feel like an idiot telling him that than youre doing a good job
Eventually you will get to a point that you will not need to give him the toy, instead you can just say "Great job!" (thats why you ALWAYS want to pair because if you dont he will learn to always get the toy. The idea is to get it down to verbal praise only). Also, MAKE SURE you NEVER take away the toy/item you are using to reinforce as a "punishment". You dont want him to associate ANYTHING negative with what you are reinforcing with. Otherwise it wont work. You are doing a great job with the time outs, just be consistant! It is tough, but it sounds like you are being a very good mommy so make sure to give yourself some credit!. PDD-NOS, ASD is a hard thing, but it gets easier and there is hope! I know i was sort of vague but if you need any other assistance, please let me know!