To build up his fine motor skills and also build up his attention span.. here are some suggestions..
Get a box of large paper clips.. Show him and then ask him to make them into one long chain.. Another week ask him to make another chain out of a box of small paperclips.. You can hang these in his room and let him attach his pictures (those crayon drawings) on the chains..
Get playdo and see how many little balls he can roll with just his first finger and thumb.. Have him try this with each hand.. also have him try it with each hand at the same time..
Have him place all of the little playdo balls in a long line and ask him to punch them down as fast as he can with just his first finger.. Then another time have him do this with his thumb.. another time his middle finger.. Keep a "Time" record to see which finger on each hand was the fastest.
Give him clothes pins (one for each hand) and have him only use his thumb and first finger, to pick different things up.. Have races with him and see which one of you can pick up the biggest (I did not say heaviest) thing with just 1 clothes pin using the first finger and thumb.. Have him sort the clothes from the laundry baskets by color, but only using the clothes pins.
Each time you do laundry, have him place the shirts and tops on hangers. Have him button all of the buttons on the clothes.
Have him separate all of the coins out of all of the piggy banks or jars of coins you have around the house. Another time have him make them into stacks of 10 of each coin..
Have him separate a jar of buttons by size.. Then another time by colors..
Get a giant piece of blank paper.. the back of wrapping paper will even work.. Have him draw the longest line. the wiggliest line.. the biggest circle.. Have him do this using different colors.
One he has about 6 different lines.. ask him to get some of his little toy cars and "drive them" only using his first finger, then thumb, on each hand. on these lines he has drawn on the big paper.. Make it a game and see if he can keep his finger on the car the entire length of the line.. If he takes his finger off, say "Uh oh" start over and see if you can do the WHOLE line this time..
Purchase puzzles. Have him put them together.. Then when he has completed 2, have him mix the 2 puzzles and so both of them art the same time.. He could [probably do 3 puzzles at the same time..
When it is dry outside side walk chalk is also excellent and he can make highways out there and drive his toy cars even longer with his fingers..