how much too small? IF you think 1/8Inch on each side of the head and at the front would help, check out
www.maltesehockey.com for their V2 SPS gel system. Its about 65 US dollars. The gell parts can be placed on top of the existing foams and will eliminate sweat running into his eyes, keep his head cooler, add a SIGNIFICANT level of protection and can be removed and added to his future helmets. I have had my set for about 4 years and my sons had his for almost the same length of time. They still look like new....
As far as from top of head to chin....take a small/medium towel and roll it into a tube. Secure with a few cross stitches here and there so it wont unroll. Now take that and curve the ends together to form a DONUT. A few more hand stitched cross stitches to hold it together. Place that on the top of the little guys head and then place the helmet on. He will have all the needed protection front to back with out the thing flopping around on his head.