Iv read that older siblings will sometimes go backwards. Some who are young enough will even demand to drink out of bottle again, sleep in their old baby beds, be carried around in your arms, or rocked to sleep every night. I thought my 6 year old was very well adjusted to our new 5 mo old baby until he surprised me the other day saying that he wished he was a baby. I asked him why and he said that then I could hold him all the time and carry him too.
My almost six year old has done the same thing....She was completely trained and as soon as we had our second child she started having accidents during the day and she has to wear a pull up every night.....I'm patient though with her, getting angry won't do a thing....I'm just letting her go through this one herself, when she's ready to control herslf she'll do it!
As a mother of two. I feel that for a child going potty is one of the only things that they can REALLY control. So if I were you I would back off and I'm sure in time he will start to do it on his own. He is obviously physically capable. He may be just trying have some control.