As the lady (i believe?) who responded before me pointed out, it depends on the degree of the sprain (assuming it is). Sprain is considered as a tear in the ligament from bone to bone. Generally, one can assume that a 1st degree sprain, your son's ankle hurts but he can walk, 2nd degree severe pain, 3rd, he probably couldn't walk. If he has a sprain, regardless of what you or he does, it will never strengthen back to as strong as it originally was. It may not be significantly weaker, or it might just be minor difference, but it will never be quite as good as the original, for the tearing ligaments that hold the ankle and feet bone together lead to instability and thus higher chance to re-sprain his foot. I know this because I probably had third degree and foolishly did not go to a doctor. What you can do though, is ensure that he goes to a physical therapist and the like, and make sure it mends properly (the physical therapist should make sure that there are no scar tissue build ups, they may constantly massage that area to break up any scar tissue before it hardens).
If its a Strain, not a sprain, that is pulling or tearing a muscle. Which is less severe, stretching (after its healed) is advised to maintain flexibility from scar tissue build up. Lastly to answer the question about how long it takes to heal, it can take anywhere from 8 - 24 weeks. Yes, that is a large range, but it heavily depends on the severity. Bones can mend at faster rates then sprains.
