Congratulations on your new family member
He's a senior so he might have trouble with kibbles if he has any dental issues. Have you had the vet take a look yet?
In the meantime, giving him canned food would probably be a good idea, since he's underweight. You really don't want him losing any more. If his teeth check out fine, you can always try a different brand of kibble to see if he prefers another flavor. To entice him to eat it, mix a spoonful of the canned with it and perhaps a touch of warm water to make a gravy.
Good luck, and thank you for giving this little senior a loving home
!!!