I know this is hard but the longer you stay at the school, the harder it is to pull away. 5 minutes of staying or 25 minutes sounds like it gets you the same results.
Get in that routine before school - let him know what is going to happen. Tell him that you will see him later and maybe give him an idea of the routine that will be taking place after preschool, "we will go shopping" or .... "I will need your help when you get home from school"... Reassure him that you will see him after school. You can even do a sticker system - one sticker each day, earned, for good behavior. When (10?) x-number of stickers are earned, you get... (ice cream after school, a hot wheel car, trip to the park, whatever the motivation is for your son.)
Get out of the habit of staying in the classroom. Your oldest might even help him adjust to this by encouraging your oldest child to talk to him about his positive experience in preschool - I never underestimate that sibling influence!
Sorry, if all else fails - he may be too young to go to preschool and you may want to consider starting up again in Fall. I had one child who was ready to go at 3 years old (Late April birthday) and the second, I sent at 4 years old (Late May Birthday), due to a speech delay. We had weekly field trips (i.e. fire station, nature center, zoo, library, etc... ) and we took time out every day to work on art projects, printing the alphabet, building, etc. Both of my children continued to be involved in programs at the library (one hour week) until 1st grade. Either way, the time was right for both of them.
I'm sure it's difficult on you to leave when he's in the throes of a tirade. I know that many of these crying jags usually end quickly, after the parent leaves, although there are some children who really are not mature enough to begin preschool at 3 years old.
Good luck to you.