This summer I bought my son's first board that was a Gong 7.6x31 MOB x 120 lt. Good price and nice look and very suitable for the short and messy italian conditions. My first son has 14 and he is very tall and bone and skin. With this board, starting almost from zero in wave riding, in a couple of months he follows me in every wave (max chest-head high). The right board has created a little avid supper. Some weeks ago I sold the Gong and I bought an amazing Hyper Nut carbon 2018 7.8x30 x 115 lt. Despite the 5 lt less it's more easy to use. With this short and large board the boy is able to catch and manage every wave because it's stable and turns quikly. Also my second one son (11yo) was able to catch little waves in very few times.
About the Hyper Nut the deck is flat and the overall quality is incomparable, also the price.... A short board for kids is easier to manage in and out of water. Also for the father.
My son is more than 1.80 and weights 60kg circa. A little bigger board it's easier at the beginning. Dont't waste time and passion in too much demanding board for yours sons.
The last but not the least a small board like this for the big old father is an amazing workout in flat water waiting for the waves. That's justify the difference in money from a cheaper board. For me seeing my son cutting a wave with me is priceless...