The problem you may have is in that calcium has to be fed in a form that will be absorbed and utilized. If your horse is already receiving an excess of phosphorus in the diet, it will bond to any calcium supplement you feed and prevent it from being utilized. So you really should be looking at adjusting your horse's diet in order to reduce the amount of phosphorus relative to calcium. The ideal ratio of calcium to phosphorus in horses is about 1.2 to 1.6 parts calcium to 1 part phosphorus. Grains are often too high in phosphorus, and forages are more well balanced. Alfalfa hay is high in calcium, which is okay since high calcium doesn't cause a problem with the ratio of calcium and phosphorus. Calcium intake should always exceed phosphorus intake.
In other words, too much calcium is better than too much phosphorus. However, feeding too much calcium can cause other serious problems, which is why the ideal ratio of 1.2 to 1.6 parts calcium to one part phosphorus has been established. But if you have to overfeed calcium to balance an excess of phosphorus, it is still not ideal.
The best form for minerals to be utilized by the body is in chelated form. This means they are bound with amino acids (proteins), and not just fed in an unbound form. Feeding human calcium supplements fails to take into account any of the factors that affect the way horses absorb and utilize calcium, so it isn't a good solution.
You can feed alfalfa hay to up the calcium ratio, but you risk an overage of calcium if you are trying to balance out an overage of phosphorus.
My point is that you are better off to try to come up with a balanced diet rather than attempting to fix this with supplements that may or may not be absorbed and/or utilized in your horse's body. If your horse is still growing, then it is vital that he has the correct calcium/phosphorus ratio to sustain the growth of his developing bones. There are feeds that have been carefully formulated based on extensive research by equine nutritionists that will meet your horse's needs, and I would recommend that you consider switching to a balanced feed rather than trying to guess at what might work for your horse.