My 10 year old son has hammer toe's and toenails that curl in toward his toe's when they grow. Is this common?

Fairly common. These should be observed until child is older. Should they ever become painful surgical repair may be indicated.
Not common. The hammertoes are the bigger concern. You should take him to a podiatrist for evaluation.
It is not uncommon. The question to me is does he have any pain with either condition. With regards to the hammertoes make sure his shoes are the correct size. Kids grow fast and oftentimes they may be in n ill fitting shoe until a parent realizes the size was wrong for a few months. Check to see if flatfooted. With regards to the nails take him to a podiatrist for treatment options.