F.B.
First of all, I think you son needs to get out of the crib and into a regular bed. I think 3 1/2 is way too old to still be in a crib. All three of my children were out of the crib and into a regular twin size bed by the time they were 1 1/2. None of my children ever climbed out of the crib either. It was just time to move to the bed.
Now, the problem you are having sounds like separation anxiety. He started to count on you coming in and comforting him. Now, he wants more. The only way that I was able to break my children of this is by simply letting them cry it out. The more you go in and do what he wants, the longer this is going to last. And, there is the possibility that if you don't let him cry it out, that he will never get over this.
Good Luck!