Hi Junebug_11, and a big welcome to the forum!
I also have a baby born this past December who we came close to losing at birth and has been seriously ill since (she’s doing well now though). So I can understand how much worry and stress you both will have been under. It sounds as though the thought of potentially losing your beautiful little boy was enough to frighten him into returning to the JWs. The ‘resurrection hope’ is the sharpest tool at the JWs disposal for drawing people into the organisation and targets people when they are at their most vulnerable. My father’s side of the family converted after having been bereft of 2 children. It’s possible that your fiancé may even be feeling guilty about having left the religion and feel that what happened to your baby was somehow a punishment for his lifestyle, or a test of his faith – that’s the manipulative effect of having been brought up as a JW can have on your thought patterns.
I’d suggest that maybe some counselling sessions that you both attend together may help you both to discuss your fears regarding your son and how you both feel about the forthcoming marriage in a neutral and calm environment before either of you commit to baptism/reinstatement and marriage.
Also, you might want to consider stopping attending meetings as before long you will find that the JWs will begin piling on the pressure to get you studying and eventually considering baptism!
Love Mummatron x