You should try to explain to him on his level that you and his dad are going to be living separately and he will be staying with you . That this decision had nothing to do with him and he is not to blame for your separating.
That you both love him very much and you are both going to take care of him and
provide for him just as you have up till now.
He will be seeing his dad regularly and that you and he will just be fine .
If you find that you and/or your son are having a hard time adjusting ,
I strognly suggest you seek an experienced counsellor to help.
BE STRONG! It will work out if you and your spouse keep the child’s best interest
Leonard M. Weiner, Esq/Divorce Solutions