Your baby is 20 months now, and you might see your baby touching and fondling with his genitals often. Don't freak out, since it is normal, absolutely normal. Your baby is on an exploration mode now. Along with exploring every little thing around him, exploring his own body will be one of the foremost.
What you need to know
Understand the fact that, one of the cardinal rules of parenting is don't pay heed to the problem if you want to solve it. Yes, you heard it right. The more you will brood on something over and over again and ask your baby not to do, you can be rest assured that it will be repeated incessantly.
Touching genitals is as normal as touching face, hand or eyes. We all do it. Just ensure that there is no infection or rashes that is making your baby uncomfortable. If not, just distract him with some toys or talk, every time you see him doing this.