If it were me...I would take Tito to another vet and asked that he be tested before bringing a new cat into the picture. Since you are unsure about what was done before this would be the safe thing to do. Although I don't know your financial situation so it is easy for me to give this advice. At 4.5 pounds I see no reason what so ever that he needs to be on a diet.

Nutros is not a bad food. Royal Canin is even better. I am not a fan of Science Diet at all.

Denyce