36/13 or 37/13 is pretty close to pro standard. Most pros run about 44 or 45/16 gearing standard (smaller these days). The 38 up front to a 13 will definitely make it more difficult for him to get up to speed, but with enough strength in his legs should allow him to reach a higher top-end speed than his friends who are likely running 39/14 gearing.

Of course, the bike possibly is a little big and weighs a little more than it should for your son which can also slow things down. I'm not sure how big your son is, but the person who designed that bike is in his 20s and is likely over 5'10" tall. This makes the bike very big for a 10 year old. Good for him to ride and get used to, but still big for the next few years.

Not sure what would be better, but he will be ahead of the game for sure if he sticks with the bigger bike now.