*sigh* Sorry things are tough right now. I'm no expert, but...
I find it helpful when DS is in a difficult emotional space, to look at what could possibly be fueling the behavior, rather than the behavior itself. Often times the behavior is simply an irrational reaction to something within (i.e., don't read to much into it, but try to alleviate the cause). As to that, just a few random thoughts (feel free to discard as needed
)...
Could your DS possibly be feeling a little jealous/resentful of his new sibling? Perhaps he sees your mom (or MIL) as a person you've set up to keep him occupied, and feels that it's just one more person in between you and him? You mentioned he's been clingy lately and having troubles with school. The hitting could very well be just another clever way of letting you know he needs some extra attention right now...
Would it be possible to have a conversation about how he might be feeling lately and what you both could do to alleviate some of the stress? I think I would try to avoid mentioning the hitting per se, but perhaps try to get underneath the issue. I do wonder if just some good, quality alone time with mom (as often as you can manage), may be just the thing. I find that there's nary an emotional ill that quality time with DS can't cure.
The only other thing I can add with regard to the hitting, is to give him an appropriate alternative. If hitting helps him to relieve stress, than by all means "hit the drum/floor/pillow/pan as often and as hard as you like. People are not for hitting... but there are plently of things you CAN hit." Sometimes DS just needs to release frustration, plain and simple. Indeed, words are nice but not always an adequate way of expressing emotion at such a young age... maybe an alternative would help?
Best of luck to you. Sounds like you have a lot on your plate right now.
I find it helpful when DS is in a difficult emotional space, to look at what could possibly be fueling the behavior, rather than the behavior itself. Often times the behavior is simply an irrational reaction to something within (i.e., don't read to much into it, but try to alleviate the cause). As to that, just a few random thoughts (feel free to discard as needed

Could your DS possibly be feeling a little jealous/resentful of his new sibling? Perhaps he sees your mom (or MIL) as a person you've set up to keep him occupied, and feels that it's just one more person in between you and him? You mentioned he's been clingy lately and having troubles with school. The hitting could very well be just another clever way of letting you know he needs some extra attention right now...
Would it be possible to have a conversation about how he might be feeling lately and what you both could do to alleviate some of the stress? I think I would try to avoid mentioning the hitting per se, but perhaps try to get underneath the issue. I do wonder if just some good, quality alone time with mom (as often as you can manage), may be just the thing. I find that there's nary an emotional ill that quality time with DS can't cure.
The only other thing I can add with regard to the hitting, is to give him an appropriate alternative. If hitting helps him to relieve stress, than by all means "hit the drum/floor/pillow/pan as often and as hard as you like. People are not for hitting... but there are plently of things you CAN hit." Sometimes DS just needs to release frustration, plain and simple. Indeed, words are nice but not always an adequate way of expressing emotion at such a young age... maybe an alternative would help?
Best of luck to you. Sounds like you have a lot on your plate right now.