My son is Potty Training, he is going to be 3 in november. He does really well when i ask him if he needs to go he says yes and we go to the potty. When he sits he freezes and he just can’t go. He just holds it and holds it until the flood gates open and he has an accident. I have tried the little potty chairs and the little toilet seat you put over the regular toilet but either way he freezes.
Try to make it more fun!!
Use a chart where he gets a sticker when he goes potty- so many stickers he gets a prize. (Let him pick the prize ahead of time so he knows what he is working for!)
Have him pick out "big boy underwear" and tell him that when he stays dry for one whole day he can wear them- put them somewhere that he can see them!!
Put Cheerios in the toilet and have him stand on a step stool and try to "sink the Cheerios". (Helps with the aim too!! 
)
Put bubbles in the toilet and ask him to pop them when he goes potty. (Again, helps with aim)
Let him go alone- put him on the potty and turn on his favorite song and leave the room!!
And remember- its easier if it is fun!!