The type of behavior you describe is not uncommon in typically developing children. However, you would like to see some more effective communication beginning to emerge. You have other children and know what two-year-olds can be like. If you have concerns about your son, absolutely bring them up with your pediatrician and ask for a referral to have him assessed. There is no reason not to do this and he might be able to start receiving early intervention which can make a profound impact if he does have any delays regardless of whether or not he receives an autism diagnosis.
Sounds like a typical 20 month old to me! Can't yet communicate effectively but know what they want/don't want so get frustrated easily. My daughter is 23 months and I can totally relate to what you are experiencing. She hits out and scratches when she can't get her own way and is also a creature of habit and does not cope well when routine is interrupted. I used to worry about autism a lot because she also rocks to soothe herself and has done this since 6 months old but there are so many other symptoms to autism that she just doesn't display so I've relaxed a bit lately. If your son happily plays with toys and smiles/responds to you, there should be nothing to worry about. He probably lashes out at his siblings just for attention (or because they are getting attention!). I've read that the 'terrible twos' can start at around 18 months - and I guess they don't call it this for nothing! Best wishes to you and your family.