Concerned if he already believed his young friends would be destroyed at 'A,' that JWs and grandparents will reinforce an already indoctrinated judgement that this would include his parents too. Beware!
A young child doesn't need to know anything else but love, joy and peace and respect for his parents. You have the right and responsibility to provide security and unconditional love for him. The JW life for youth is to feel always guilty and feel love from God and JW organization is only conditional and that God will 'destroy' those that disobey. Is this what you want for your son? This is not healthy for a young child to grow up with this complexity and contradiction.
You can provide simple little Bible story books if you wish, for his age appropriate. WT programs don't have age appropriate programs. Keep him mentally challenged for his age. Have teaching dvds and books available for him. Find some friends for him through school or good neighborhood friends.
Please don't give up on 'your' parenting concerns and abilities, over to your in-laws. They had their chance and opportunity with their son. Must be a reason why he has a more 'moderate' belief concern for his son. The WT is not a 'moderate' religion but it is extreme in manipulative control.
I had 5 children when I realized that the WT is not true. It was gradual for me. Kids adapt very easily. They were happy with the changes. Sports were big for them. Then there's music and arts too. Having school friends was great for them. School work and learning was high value, encouragment all along the way for college some day. There's plenty to be busy about and having a positive attitude along the way. There are the normal day to day things in life, on what is right or wrong, appropriate or not appropriate,, good or bad. You, as the parent, are the one to teach that, instruct, along the way. They all have put themselves through college now. Some are married and & parents now. I love being a grandparent and gladly trust their parents to be the parents now.
So many best wishes to you both!! YOU can do it!