Well this question is not easy to answer !
It depends as always on so many factors !
What is not going to work is him gaining super fast like 20 kg in 6 mounts ...
But like 1-2 kg a mounth should be possible

in the beginning !
I guess you are not living to gather,
So focus you activities around food, always have snacks around, cook for him, order pizza ( check for discounts like 3 for 2 )
Make breakfast bacon n eggs toast and a banana milk shake ( 200 g cream 1-2 ripe bananas 200 milk and some sugar30 g blend with ice is already is a solid 1000 kal.)
Go on vacation —> all inclusive
Take the activ part in the bedroom !
If he is doing sports in a club try to convince him to go on dates instead.
Drinking alcohol, Aperitifs like aperol increase the Appetit !
Large plates, large breakfast, in the restaurant order large portions for you self, than say you are full and make him eat the rest.
—> so many options always depands if he is reacting like you want him to 😅
If he is diat continuous It will probably not work !
Alsow after he gained the fist 10-15 kg you should come out about it at least partially