The answer your second question is no. Preschool is not for parents it is for children. Not only will the child be learning to socialize with other kids, s/he will be learning to thrive without mom & dad. If you are not ready to separate from him or vice versa then find a mommy & me program where you would attend together.
About when to send him, it really depends on 2 things. #1 Is he developmentally ready, and #2 The school's policy. To know if he is developmentally ready you need to assess the following: Is he relatively independent, Can he separate from you easily (without attachment issues), does he have the ability to endure 2 - 3 hours, can he follow routines/directions, Would he be able to follow the routine, sit in circle time, etc.
You need to also find out the school's policies. What age do they begin accepting students? Will he need to be potty trained?
My daughter's first preschool experience accepted students beginning at 2 years 9 months of age. The children did not have to be potty trained. And I knew she could handle the 3 days a week 2.5 hours schedule.
Our current preschool, begins at age 3 but may accept a child a couple months shy of 3 if they are potty trained. It is 2 days a week for 2.5 hours. The child needs to be potty trained or at least on their way - they do not change diapers (Mrs. Jenn correct me if I am wrong :) )
My youngest will be 2 in February and although I am hoping he will be potty trained by September I have no interest in sending him to their 2 day program just yet. Instead I may join the mommy & me group there (upstairs from the classroom) to keep up the routine we have maintained for the past 2 years so that his transition when he is 3 will be smoother.
I hope this helped some.
A.