Any seat will work as long as it's the right size for him. Don't use a pee pee guard, just throw it away. They hurt and make it harder to sit down.
Toilet seat inserts are for later after they're already trained. They aren't secure and little ones feel like they might fall off.
Make sure the opening is wide enough and cushion seats are best. Keep in mind that it needs to be easy for you to clean. I have the safety first with cushion seat.
My son was trained at 2.5 years old. Don't listen to people who say that's too young. We casually start introducing the potty at 18 months and one day he just figured it out. We had lots of accidents that summer but he was independent day and night by 3 years.
The No Cry Potty Solution book was a great help in setting expectations and offering useful tips. Best tip, treat it like teaching to use a spoon. It takes time, there are lots of accidents but he will eventually learn. Don't every nag or get mad. Set backs are natural and correct themselves with time.