My case is the same, don't know how to teach him.
When I got the potty near him and he will refuse it.
But when elder brother need to 尿尿, he will get the potty near him.
just want to share, but don't know whether this is the main reason. My son 肯企響樹尿尿 lar.
After 2 times he have shower bath together with Dady and Dady tell him to 尿尿 during the shower bath.
You may try that. Give lots of water to the boys and when they need to 尿尿 and go with him to have 企響樹 shower bath.