L.,
It is very common for a child to regress with certain behaviors. Some will start sucking their thumb, pacifer, and start wetting themselves. It is a huge stress for a sibling to accept another, even if they generally love them dearly. You do not have to treat him differently, but he does not have full attention anymore. Which is very common for the oldest to do this, because they are an only child for so long and then have to share the spotlight. He will outgrow it, may take a while and he may not know he is doing it subconciously. My son will be 4 and has 4 month old twin brothers and even though he loves them (hugs and kisses, and plays with them a lot) he will do things like, my turn now. I need changed. Just let him know, it's alright, accidents happen, and we might be disappointed but it is not acceptable to lie, or fib, (however, you let him know lies are wrong)and that if he does lie about it, he will have time outs, etc. Let him know, that as long he tells the truth about it, then lets work on it and you will not get in trouble.
Hope it helps, there is no set time but regression is normal for kids with siblings.