Just help him.
Comparing my daughter and son... my daughter started dressing/undressing herself very early.
My son, did not.
For my son, it was his coordination/fine motor skills. Even if he is really athletic and coordinated that way, per dressing/undressing he was all thumbs.
So I just helped him... while "practicing" with him.
Showing him how.
I made it fun.
My son is very independent... so him dressing/undressing himself had nothing to do with his 'independence." It was just a thing, he hadn't 'mastered' yet.
Plus, my son LIKED to wear the SAME thing, everyday. He had his favorite t-shirts.
In any case, don't force him or scold for it. It is just a basic skill... but for some kids, they just do not know how, yet nor have it fully mastered.
Just help him.
Show him how.
Practice.
There are so many different ways, to put on or off a t-shirt. Just show him the easiest way to do it.
My son, he will take his arms out first. By pulling on the sleeves and slipping his arms out of it. Then he will pull his shirt off over his head. Versus for ME... I simply from the hem of the t-shirt, will just pull it completely off over me and off. All at one time. My son, couldn't do it that way. No biggie.
My son is 4. We first practiced with him about pulling down his pants. When he got that, we practiced pulling UP his pants. That was harder for him. So we took more time with that. Only when he was about 4 (he is 4.5 years old now), he could pull off his own shirt.
So you see, it was in stages. Of accomplishment.
My son however, has great coordination and fine-motor skills with so many other things. Just with dressing/undressing, he was all thumbs.
Its really not a big deal.
Don't go by his 'age.' Go by HIM.
Just practice but don't make it a pressured thing.
Kids that age still need help, with dressing and getting ready.
all the best,
Susan