What kind of pen grip is he using as different ones work for different children. One of my DC uses a crossguard grip which has two sort of covered 'wells' for their fingers to go into and a strip in between to stop him crossing his fingers over one another. Another of my DC uses an ultra-grip pen grip which is a contoured lump of silicone to make the pen chunkier and therefore easier to hold.
Thera-putty is great for strengthening hands and fingers. It's a stiffened putty that gives some resistance when squeezed so helps work all the little muscles and ligaments supporting the joints. It comes in three strengths based on colour- yellow is the lowest/softest, red is the middle range, and then green is the toughest. My younger DS uses red and the older uses green. There are lots of exercises that you can do with them but the most basic one for strengthening fingers and hands is to tear off a fist-sized piece of putty and roll it into a ball, then cup the ball and use your thumbs to press a hole in the centre followed by using your fingers to pinch around the edges to make a sort of bowl. You then poke holes in the base of the bowl with an extended finger and put a bead or button inside each one, fold the sides of the pot over and roll it back into a ball. You then swap it with another person (I do this together with the DC) and you both stretch, squeeze, and pull the dough until you have found and removed all of the beads/buttons. We do this twice a day.
Games involving tweezers and using a pinching action are good. Things like picking up coins one at a time and putting them in a piggy bank using only a pincer movement, timing it and then trying to beat the record. Using tweezers to move beads or buttons from one bowl to another, there are also games you can buy for this such as a pie sorting game with little fruit that you move between segments using tweezers.
Threading games like thread beads on a string, threading a lace through holes punches in a piece of card, threading bobbins, etc.
Lego is another good one, the action of clicking and unclicking the pieces.
Any sort of squeezing action such as Playdoh, kneading bread, kinetic sand, sponges or flannels in the bath to squeeze and wring out.
Practice with letter formation such as painting on a wall or blackboard with water, drawing with a finger in a sand or flour tray (an oven tray with flour or sand on).
Rolling out Playdoh into ansausage and then using a knife and fork to cut it into pieces.
Using only the thumb and first two fingers of both hands to tear a piece of scrap paper into strips, then small pieces, and then using only their dominant hand to roll the pieces one at a time into small balls.
Playing button games. We have some 'sandwiches' that I made from pieces of fabric and felt cut into the shapes of bread, a piece of lettuce, slices of meat/cheese/tomatoes, etc. The bottom slice of 'bread' has a button stitched onto it and all the other pieces have a button hole. DS has to take the sandwich apart by unbuttoning it one layer at a time and then re-assembling also one layer at a time.
Check his posture too when he is sitting as this can make a difference. His feet should be flat to the floor with his knees bend and his thighs flat. If he can't touch the floor then he needs something to rest his feet on like a box or a step. If his knees are higher than his thighs then the table needs raising. If he is a fidgety sitter then a wobble cushion or a move n sit cushion can help. If he's a foot shuffler then a the band around the front chair legs for his to rest his calves against can all help. Another thing that is sometimes needed to support writing is a writing slope to put their work at a slight angle which makes it easier to apply the correct pressure for writing.
We have spent a lot of time at Occupational Therapy over the years!