His stools are so hard he cries. Now his legs seem OK, but he catches lots of colds and every time his lungs get infected. According to the growth chart we use in the United States, your son would be in the 25th percentile for weight, which means that 75 percent of the other boys his age would weigh more than he does. However, that does not mean he is underweight. It sounds like your son may have been born a little early, and sometimes it takes these babies longer to mature and they are more prone to catching respiratory infections.
Calcium supplements can cause constipation.
If your son has not had a good physical by a pediatrician, have him examined. The doctor may want to do some lab work to be sure that your son's immune system is working adequately. You didn't say anything about his activity level, but this is another area a pediatrician would want to examine to be sure that your son's heart is working well and pumping blood to all parts of the body. If your son does have rickets, it can be treated. It's important to find out if an intestinal problem or a kidney defect is interfering with mineral absorption.