C.K.
I hate to say it, but I think potty training is one of the toughest things about being a Mum.
I would use the underwear, no diapers - especially through the day. Take him every 20 minutes or so to the potty and get him to sit on it - whether he does anything or not. Set a timer for the 20mins., incase you forget. Are you getting him to sit rather than stand? It's ok for them to sit till they are good at aiming. Have the potty handy. Eventually you'll hit the jackpot and he'll get to understand. Perseverance and lots of patience is the key, it isn't easy when you have a second child too. Even though it feels like all you are doing in the beginning is going to the potty and sitting, it will be worth it. There will be several accidents, and the hardest part is if you have to go out during the day. If possible stay home in the initial days. Good luck and keep your cool - sometimes we take things we have learnt for granted, but we all had to learn things from scratch!