Most kids do better with the potty but some train successfully on the big potty. I bought the special seat that goes on the toilet but it was not stable and scared my son. Since he tells you when he goes in his diaper then why not ask him if he wants to go on the potty? Telling after making pee or poop is a big sign of readiness. I would not just put on underwear right away though unless you are ready for a lot of mess and a very confused ashamed child. If you potty train, commit to it and go into it full speed ahead. Use pullups but put him on the potty (or toilet) every 15 minutes for the first day or 2. Give him a reward (like an m&m or gummy) each time. Then after 2 days increase the time to 30 minutes but only give a treat when he actually makes something in the potty. If he asks to go and them does, give him a bigger treat. Wear pullups in between but make sure to discuss "accidents". Don't make him feel ashamed, just talk about it matter of factly. After a week, he should really have it down. At this point, put him in underwear and let him have accidents. If he is having more than 1 or 2 accidents a day then you need to stop training or go back to pullups and go intense again like the beginning. If your child is truly ready, he should be trained almost perfectly (all kids have accidents, especially at night) in a matter of weeks.