I have a soldier with an ETS date is in 20 days, and the command wants to give him a PT test. Does the soldier have to do it/pass or fail?

In the short term, he could face UCMJ if he refuses to do it, regardless of whether the test is lawful. But this sounds like harassment; I cannot think of a single good reason for his participation. He should have had his physical by then. If he is injured, his ETS date could change. And in the bigger picture, he could go to the VA in three weeks and report a major injury and say it happened due to command harassing him by forcing participation in a PT test conducted after his ETS physical. Think of the can of worms THAT would open. I would tell him to take test, but privately imply to him that a Smart Sapper would do as little on that test and any possible PT sessions as he thought wise.

In Germany, we had a guy that was ETSing and Top made him take his final PT test. He had been known to do 100 pushups, 100 situps and 10-2 miles. He just told them 10, 10 and 20 meaning 10 pushups, 10 situps and 20 minute 2 miles. He did just that. He even smoked a cigarette while running the 2 miles.

While unsure of what policy is, it would seem to me, that this is a case of pettiness or harrassment.
When I only had 4 weeks left in the Corps, I had my discharge physical. At that point, the company 1st Sgt decided he wanted to take the unit on a 26 mile hike with pack and 782 gear. I simply let him know that it wasn't going to happen as my discharge physical was completed and I would not participate. His response was petty. "If you were a good NCO, you'd be marching with your troops."
My response was simple, "If I was a good NCO, I would have been promoted to SSgt."
Best of luck to you Sgt G. Hopefully you have made good friends while serving. And, hopefully, your brothers in arms will remember that the politics that causes many great soldiers, marines, sailors and airmen leave the service is not the end all and be all.
Be proud of what you did and carry on.

Dont know about the Army, but in the USMC, you may be directed to take a pt test at any time. However, you cannot be made to take it for score. But if you fail it does mean you are under evaluation for being put on a pt program.
However, if a Commander did this with that close to an EAS, it would likely result in a request mast and the PC getting in trouble.

Some people really should not be in command slots.

Its either harassing or they're too stupid to know they don't need one from him. I actually had a doctor I worked with in this almost exact situation. He was enough of a smart-ass that he showed up, did 1 push up, 1 sit up and sat down just across the starting line for the 2-mile run, stood up, said "okay, thanks" and went about his day of getting out of the Army. He got out, went to a surgical residency and he is much happier. How do you think that failed PT test affected him or his final OER...?

WHY?????

EEO complaint, this is what happens when education based leadership is given a higher stature over God’s Holy Spirit based Leadership.

He shouldn't have to but might be better to check with command

As a former Training NCO, I would look to see if he/she has a passed APFT/ACFT within the last year, if he/she does, let him leave without issue. But if he/she hasn't, you are required to chapter him/her as not meeting standard. If he/she decides to come back in, he/she would have to pass one in order to be accepted.
It might end up being a reduced discharge. (General under Honorable circumstances) and he/she would have a conditional entry code on their DD214.