I save my cat when he was almost the same age as yours.  Me and my wife took tremendous measures, having his best interests at heart. 

For neutering,  best determine the gender first. 
If male, then the need is a tad lesser. Ours is not neutered yet too.

We fed our boy milk replacement on his 2nd month age,  then starting 3rd month we gave him wet food.
Later on we slowly mix,  and gradually kncrease the amount of kibbles and reducing wet food as time goes on.  The process took about 2 or 3 weeks.

Now the guys 6.5kg
Before:


After: