There is a fact about dogs that they will love one member from the family more than the rest of the members. You yourselves will notice their affection towards them. If you ever want to see just call that person in front of your dog and he will search for him everywhere or even if your dog is in deep sleep he will wake up if that person has come. He will be sad in his absence he will even cry. It could be quite difficult to lift his mood or even get him eating as he will not eat until that person is present.
Since he is clearly not in a very good and you will have to make up for this so you should feed him his favourite food he will not eat that in the first attempt because of the bad mood but he definitely will in some time. Do not give up on him too soon. You can also give him some biscuits he likes.
BE WITH HIM:-
Don’t leave him alone as he is already feeling too lonely in the absence of his favourite person. Keep him entertained by playing games like fetch and hide and seek with him this will take his mind off at least for some time. You both can also sleep together and I assure that is going to be the best and the cosiest sleep you ever had.
VIDEO CALL/PHONE CALL:-
If it’s possible you can call the person he is most attached too and make him see or hear his voice. Initially they will get a bit aggressive but will calm down . This will make them feel as if the person they are missing the most is always around them. After that he will be less of a fuss that he was.
TAKE HIM FOR A WALK:-
A short walk would be a great help for both of you as he too will get a change and will meet his friends that might lighten and brighten his mood a bit.