Mostly, I am in agreement with the comments by others here. For the past 28 years, I typically start children at age 5 with four-year olds being the exception if they demonstrate an awareness of most five-year olds, or older.
My twin boys are 8 years old, and my youngest is three. I started my twins at age four, but they did not promote until they were 7. It will vary for each individual. I do not want to push my own children in the learning or promoting process, but I started them when I said they were ready, and I will require them to train regularly until they are eighteen.
Children are never too young to learn, thus we can teach them anything in an informal setting. Under four is usually too early for a structured class of any significant length. My boys watched me train at home from the time they were born. My twins were just walking at 9 months when one of them set his milk bottle on a railing at his head level, and kicked it off with a perfectly executed roundhouse kick. I never "taught" it to him, but he must have picked it up by watching me do forms.
i just get worried he is being bullied at daycare
This comment worries me though. I believe that children under 5 should not be expected to have good enough skills to defend themselves, even against other kids their age, nor the proper judgment to use it appropriately. If you are concerned about a two year old being bullied in daycare, then there either needs to be better
constant adult supervision, or take your child to a different daycare facility.
My personal opinion is that I would never leave my child in the care of others (Close relatives as an occasional exception). I would rather quit my job, and raise my own children than to leave them with someone I do not trust to protect them from bullies, weapons, or molestation. There are too many stories about abuse, neglect, fights, drugs, weapons, etc. If you are concerned about your child's well-being, take action, but I do not believe that teaching a two year old self defense is the answer.
Just my opinion
CM D.J. Eisenhart