There are a lot of fun fine motor skills you could do with your son.. He will think you are just playing with him.
At this point, let him use both hands if he is not showing a preference. Could be he is ambidextrous..
Playdo
Get one container and have him make as many small balls with the playdo,. They do not have to be perfect.. Show him how to make them with 2 fingers and also the palm of his hand.
Have him line them up in rows.. You decide.. rows of 5 or 10.. Once they are all made and in rows.. Have him punch the first row just using his thumb. The next row, just using his fore finger.. continue and then have him use his other hand,. each finger.
Another time, have him stack the small playdo balls in towers.. One on top of another.. Again.. after they are all stacked have him squish them with each digit.
Another time, cover your entire kitchen table with plain paper. I used to have odd pieces of old gift wrap that I would turn over so that the plain side was showing. Have him pick 1 color and draw a really long line. Remind him not to raise the color off of the paper. If he does lift it. Give him another color and see if he can make a longer line this time.
Then have him use a 3rd color and have him draw bid curving lines. they can cross over the other lines.
Get his hot wheels cars.. and have him "drive" one of the cars on the first line.. Have him only use his forefinger.
Then have him drive the second line, then the curvy line. These are now like streets for his cars. Have him practice with each of his fingers.. including his thumb.. on each hand.
Another time have him use a marker or colored pencils.
Scissors are great to have him practice making collages. Give him a list of things ro search for in old magazines..
A car, a flower, a person with a smile, a baby, a sad face..
Then give him a glue stick and have him glue them onto a box.
Get a hole puncher and make a picture on a piece of string paper. with the hole puncher.
Have him use a piece of yarn with one end tapes.. to look like a needle.. Let him pretend to sew..
Every chance you get, have him button up shirts and pants. The clean clothes from the laundry.. make it his job to button all buttons, snap all snaps.
Give him a box of the big paper clips and have him make a chain out of them.. Then sometimes, have him put together a chain of the small paper clips.
Give him some clothes pins and have him pick up different things, only using the clothes pins. Maybe have a race with you and dad and see who can pick up the largest item with the clothes pin.
Let him practice with Tongs from the kitchen.
Show him how to line up dominoes. Then show him how they all fall down in a pattern.
Stacking blocks..
Stacking marshmallows..
Stringing cereal like cheerios to make a necklace.
As you can see these are all things to help strengthen his hands, wrist, and fingers. He will just thin it is playing..