Can I make a POA for my son with Down Syndrome, he is 20 years old?

A POA can only be assigned by a competent person. You son may not fall into that criteria.

You have guardianship. That is all you need. Guardianship covers a lot more than Durable Power of Attorney. You already handle his financial and Medical and more than a POA covers.

Legal Guardianship can provide various types of authority to act on behalf of the Ward. I might take that court order, to an attorney in your jurisdiction, to have it reviewed. You can inquire if anything else is needed. In order to execute a POA, the person would need to be competent. So, that might be in question with your situation.

Some things that I'd ask about is if your Guardianship is permanent. Unless, something caused it to stop, it should provide you with very broad authority. There are also several types of Guardians, like General, of the person and of the estate. It varies by state, but, I'd check with an Elder Law attorney in your state, since, you should be informed of what rights you and your son have. You can also read the court order that gave you Guardianship. An attorney could help with it though.

I have an adult son with a brain injury. I got POA for him several years ago. There was little to it. Just filled out the paper, he signed it at the bank in front of their notary. We got state specific POA forms online.