L.T.
While I understand your concerns for wanting to keep your son in par with his peers, I don't think it is anything to worry about at this point in terms of your son's development. And if he doesn't master it by the time he starts school next year he will not be beyond help. Many of us moms 40 and older didn't really learn these skills until kindergarten. We weren't required to know as much before starting school as kids are now.
Aside from practicing writing what about having him do activities to strengthen his muscles and practice dexterity: molding clay, lacing cards, stringing beads, finger painting, gluing dried beans/macaroni/cereal into mosaics/pictures, cutting with scissors, buidling with Legos, playing with stickers, playing Lite Brite, picking up objects with pliers, making his own pbj sandwich or eating dinner with chopsticks.