Don't feel in a rush about it.
He is a boy and also has SPD etc.
It will come.
He will be fine.
My son is 4, he can dress himself, but still, boys can be later in "fine" motor skills.
Differentiate between 'gross' motor and 'fine' motor skills.
Most kids are better at one than the other.
At that age, I still helped my daughter. My son too.
Just make sure you tell him the 'tag' goes on the back.
So they know which side is front and which is back.
And, per this age and toileting... just practice with him about 'how' to pull up and down his pants/underwear.
Use clothing with ELASTIC waistbands.
Much easier.... and more manageable for young children.
Your son is young.
I wouldn't fret over it per him dressing himself or not fully, yet.