I know most doctors will recommend that you wait to start solids until 6 months, but my kids wanted FOOD at 4 months. Neither of my boys was a fan of rice cereal, though, so after a few attempts, I would switch to oatmeal. When my oldest was 4 months old, he desperately wanted a banana that someone had. I let him have a taste and he polished off almost 1/2 of it! He was one happy boy after that and was hooked on solids. We never had any problems because of it. By 6 months, he could pick up Cheerios and wanted to eat everything we ate. It was hard to keep him at bay when it came to food. =)
I think it's TOTALLY fine to start at 5 months, especially since your son is big and healthy, but I would start by diluting it a lot until he gets a feel for it, and only wait a while before introducing more new foods if at all possible.