You have a tough choice to make
1) fade slowly and risk your son becoming hopelessly indoctrinated with fear and intimidation
2) fade abuptly and risk damaging your relationship with your husband beyond repair
You are in a similar position to me a couple years ago. I tried to talk with my wife about different things but all I succeeded in doing was showing my cards. She was all in and would entertain no such idea of daring to question anything that is taught. It all came to a head when my 5 year old told me that I hate Jehovah and love Satan. I had had it. For the sake of giving my children a normal childhood free from the mental shackles of a dangerous and controlling cult......I decided to hasten my fade and up the anty. I began talking to my children and telling them that the bible does NOT say that birthdays are bad. I began to reason with them based on logic....whereas before I basically kept my mouth shut and allowed my wife to indoctrinate. My stance infuriated my wife and I knew it was only a matter of time before the shit hit the fan. That happened quickly. My wife left. Custody is still not settled but at the very least.......I can and have reached my children in a number of different areas. They both want to celebrate their birthdays. Neither of them thinks the idea of God killing billions of people for not being in a particular religion is beyond unreasonable and cruel.
For me.....my wife basically gave me an ultimatum. Shut my mouth, watch my children grow up as JWs and maintain my marriage........or try to give my children freedom and watch my marriage crumble. It was tough for a while......but I made the right choice. Kids need a normal childhood. That's not to say that they can't figure things out for themselves when they are older. You did. I did. A lot of people did. But dammit.....how much better would it have been to have had a NORMAL childhood?
I don't know what your future holds, but you may be in for a tough decision in the near future.