Hi,
Many times the word 'small' in this context mean 'not large'. So long as the organ is fuctionally useful and healthy, as has been already proved in this case, I do not think it should be a point of worry. Many times in this Forum people have opined that it is not the size that generally matters, but how one uses it is important. A healthy organ, though relatively small is always better than a nonhealthy bigger organ in this case. They say, genetics does play a part in the size, provided, it is a family trait acquired by his father. But if however, in his case it was due to some growth problem during puberty, then it should not affect, I feel.
In any case, your son is only ten years old, and you need to wait for his puberty to go through. In the mean time it is a good idea, if he can be told now that size of the organ is not very important in sexual relations, as compared to the health of the organ.May be, you could take him now to his doctor, and get his opinion as well, and at the same time, request him to instill some confidence in him so that just in case there is a size problem, he will not worry much about it. Make sure you take extreme care in giving him balanced food,rich in proteins and micronutrients from now onwards, so that he will be able to grow to his maximum level during the ensuing pubertal days.
Best of luck, do not worry, and pass on your worries to him too.
Solace
I don't know a lot that one can do...at his age, "going in" while sitting down is fairly normal.
Have you seen a doctor? Urologist? They may have some possible treatments and can do a workup to see if there is something medically wrong causing the situation.
Jim