The mucus membranes in his nose are irritated from a cold virus. There are blood vessels in there too so it's easy for irritation to cause bleeding. It can happen with a lot of nose-blowing too, although most 19 month olds can't do that effectively. The solution the other post is taking about is a saline nasal spray - Ocean is one brand but there are others. Store brands are okay. You just squirt it into the nostril to help moisturize.
Some people use a Neti pot very successfully but I think it's almost impossible to use with a little one who can't blow his nose afterwards. It's better for older kids & adults.
More fluids will help but I've found the best solution is a terrific supplement designed for kids called Reliv NOW for kids. You can't get it in stores which is good! You get it from a trained nutritional consultant who helps monitor you and gives you the support you need. Helping kids avoid colds entirely is a great thing! My son is older but hasn't been sick in 3 years, neither have my friends' little ones.