My immediate thought - I'd be fairly worried if a teacher hadn't noticed that a child had left marks around another child's neck. Supervision....!
My Ds went through a phase a bit like this. He was more bossy/aggressive if he had too much screen time, if he was stressed, or he'd eaten sweets/processed food.
We made a rule that holding hands was ok but being rough, pushing, hitting, kicking was not. Ever. And then reinforced that over and over. What is your Ds's favourite thing - TV, tablet - every time he is rough, remove it for a day, if he does it again, two days and so on. Explain clearly & calmly. Then talk about something else, something positive. Don't blame your ds, or nag him. But no giving in either.
And lots of fresh air and exercise so he's too tired to want to rough&tumble. Bath & early bed routine. Which will give you some chill out time too.
If you have a childminder or afterschool care, enlist their help.
I completely get your anxiety over him having friends but you stressing won't help, because he can sense it and get stressed too. Why not invite one of his classmates over on Saturday. Plan something - Take them to the park or exploring the woods then back for snack. Give him a big cuddle if he is good, tell him what a nice person he is. Build up his confidence a bit. These things take time so decide on your strategy and stick to it firmly for six months.
At 5 he is old enough to know he's being naughty, and once he realises life is nicer when he is gentle with people, I'm sure he'll behave better.