If you get severely constipated, then stools can compress the sciatic nerves that run down the leg and can cause numbness down the legs. Once you have been able to open the bowels, then this should resolve.
Do you know why your grandson has been so constipated for so long?
Also, taking Milk of Magnesia every night may not help or the relief maybe temporary, as taking this on a long term basis will alter the stomach acid and this makes it more alkaline. By altering the stomach acid constipation can become worse.
You probably need to get him on to Metamucil twice daily. This is a water soluble fiber supplement that can safely be taken twice daily and is as good for constipation as it is for diarrhea. Ensure that you take enough water with it though.
Try eating live yogurt or taking Acidophilus caps. This will help to rebalance the good gut bacteria and could help the constipation. Also Magnesium supplements could be useful in resolving constipation.
Also, look at his diet. Try and cut out junk food, chips, soda, caffeine and ensure that he eats plenty of fruit and veg and carbs. Add in prune juice to help.
If he needs to open his bowels, then glycerinne suppositories rather than Magnesia will be better. You can also make up a baking soda enema yourself if needed.
Try these suggestions and see, although I do think that continued Magnesia use is not the answer.