I'm not a teacher, but I've been reading these forums for a few years (my DC is in Y5) and I get the impression this is quite common, particularly with summer-borns, and particularly with boys! If you do some searching on this forum you'll probably find a few historic posts on a similar theme, with some great advice and reassurance.
People tend to suggest not panicking or forcing at this early age. You don't want to put him off writing!
Building gross motor skills will help him with holding himself in a good position for writing, so plenty of running, climbing, monkey bars and suchlike to strengthen the core, back and shoulders. Swimming?
Also other activities for developing fine motor skills, such as play dough, threading beads, Lego, Aquabeads - anything a bit fiddly to do.
Writing/drawing with chalk on the patio over the holidays?
Some find it easier to write 'upright' so if you have a whiteboard on a wall, or maybe stick large paper onto a wall/window and encourage him to draw/write that way.
Postcards to relatives, if you go on holiday.