3 weeks is not long....
Just try it... have him sit on the public toilet... since he can't reach that height standing.
Have an extra pair of clothing with you, just in case he has an accident or refuses to go on the public toilet.
For me, I took it in steps... first them being solidly potty comfortable on a toilet at home... when out, still diaper/pull-ups on. Only for going out. My kids know that. Then, once I knew they could hold it long enough and was mature enough, then no diapers when going out.
I also had a car potty... which is a life saver or when on the road or in traffic. I got mine from www.amazon.com just put in the search word "on the go potty." When my daughter was scared of the public toilet, I used the car potty as well.
For my son, he is in the process of potty learning... but I do not yet take him out, just in underwear. But when he is ready, I will. He's not there yet.
For my daughter, as well, once I felt she was ready for going out just in underwear, and using a public toilet, we went out, for short periods... and then I had a folding toilet seat to use on the public toilets.
Its a process.... in steps.
And some kids don't like the automatic flushing toilets.. .it scares them.
In time.
All the best,
Susan