Hi C.,
Head size does not really matter in what i am about to suggest to you. I would suggest that you have your son assessed for (what seems to be little known) DYSPRAXIA My son has this disability and it involves the delayment of BOTH fine and gross motor skill abilities, it can also affect memory, sometimes it will affect speech. Dyspraxic children can also be quite LITERAL but on the bright side, Dyspraxics are almost across the board above average or higher in intelligence. So it really comes down to a childs mind not processing the info to it's body in the manner necessary. Depending on the degree of motor skill deficiency, occupational therapy may become necessary, I in no way intend to worry or discourage you, but if it turns out that i led you toward the correct path, let me reassure you, my Dyspraxic son is 11 now and managing in this world just fine, so, no worries!
B.