If you didn't mention that DS has periods of being sunny natured and well-behaved, let alone have a DC2 under 6 months of age, I might be more concerned.
Are you worried about his hearing or sight, if so raise that with your health visitor or take DS to your GP. At this age it is as likely to be wilful unwillingness to pay heed and stop playing or follow orders. As amazingmum says, if in doubt, take him along to the surgery.
Toileting issues, that is such a landmark, every parent recognises that, if you've come through that, one hassle less!
With the best will in the world the bulk of your attention has probably been on DC2. DS suddenly looks huge in comparison. He is more mature by dint of being older but he is still only little himself. He needs to learn to be polite and unselfish and thoughtful where others are concerned. If he has become territorial or possessive he is venting aggression that could come from feeling insecure.
Make the most of any good behaviour by praise and affection. Positive reinforcement persuades your child he gains from being consistently 'good'. Don't just wait for him to start playing up to pay him attention. Something as simple as calling across the room, or just hold his hand, cuddle him, stroke his hair as you walk past, talk to him not at him. I agree (again!) with amazingmum, a bit of 'babying' can really help.
Quiet time spent feeding your youngest can also be quiet cuddle or read-a-story-time with DS. Or CBeebies or whatever. 20 minutes quality one-on-one time with DS while DC2 naps or DH takes over won't chip into your day too much but he'll get a real boost.
When he acts up, don't use a blanket term like "Don't be so naughty!" Be specific, tell him what you don't want to do. Try using sentences starting with, "DO..." instead of "Don't..." to vary the message. A good tip is to move the child from the area preferably to a quiet space where he won't get wound up being told off in front of others. If picking him up or leading him to a naughty step doesn't work, take yourself out of the room. Anything he uses as a missile, remove it. Any kind of attempt to control by spanking is going to backfire, DS will think aha that is what I'll do too, but he won't use it in moderation, he won't discriminate who he hits.
Let him know you understand when he is upset or cross about something, sounding sympathetic and acknowledging he feels annoyed or fed up, then jollying him out of it works better than telling him off or disregarding his mood.
I'm not saying, never give him an order, or, analyse every detail to death.
I have heard people say you can't reason with a child that age, they don't listen anyway. Personally I found that if I put into words something of what my DCs felt they were comforted by Mum knowing what they were cross about. Then I could suggest a way of sorting the situation out. Sometimes it was like a stuck record "I know you don't like broccoli we'll just cut up beans instead and we'll put that broccoli on another plate. Now how about you eat up the rest".
The difficulty is judging when to pick your battles. Sometimes "giving in" over broccoli is actually a tactical withdrawal so he'll eat the rest of what's on his plate.
Btw don't be too embarrassed to apologise if you get caught out doing something wrong or in the heat of the moment, little children appreciate honesty if you say, "Mummy got it wrong, silly Mummy" or "You're right, Mummy won't do it again".