Hi M.,
I have fraternal 3 yr old twin boys myself. So I know what you are going through. Even though they are twins, remember that they are no more alike than any other 2 siblings. Your daughter trained first because girls usually train quicker than boys. So fon't feel that her getting potty trained is any indication that your son is ready too. He will be ready in his own time. So I have to agree with what his doctor and teacher on this, even though it seems like it is taking so long.
We are potty training currently ourselves. My suggestion is to continue introducing the potty issue. Make the rewards very clear.
We started off with: If they went potty, they got a piece of candy(They turned 3 just before Halloween), a sticker on their shirt to show off to each other, and a sticker on their chart to show how well they were doing. And the big reward for being potty trained was going to see the movie Cars in the theater.
They were struggling still with staying dry and unsoiled, but would still try to go when asked. So we switched to candy for dry/unsoiled underwear and stickers for #1 & #2 in the toilet. And since Cars was no longer in the theater I bought the DVD so they could look at the DVD case and have the reward in their hands to make it more real. We have not broken the seal. I have them try to go potty about every 40 minutes now. I tried to let them tell me when they had to go yesterday and it didn't work well. So I am back to having them go every 40 minutes until they are ready to try on their own time table again.
Hope this helps.
G.