Consider buying a dry dog food as suggested by another responder. Also look for food that may indicate that it is for an adult dog and possibly one that is for an indoor animal - if that is the case. You do not want an older pet to get fat as it will lead to many health issues. Regular exercise is important and therefore one or more walks each day is highly beneficial for long life.
I'd suggest changing to dry for the sake of his teeth, unless you brush his teeth daily. Any good quality dog food formulated for small breeds would be fine. Read the ingredient label and try to find one that has meat or poultry as the first ingredient.
The best dog food to feed a 5 year old Chihuahua is one that is the major dog food brands which is semi-moist. You can also feed him dry kibble mixed with canned. Look for brands like Solid Gold or Science Diet where they offer food for juvenile canine in 'small bites
Best dog food for your chihuahua would be EVO. most chihuahuas won't turn it down. They are picky eaters. I have found the small bites work best for them. And keep lots of things for your dog to chew on. Very important for chihuahuas. They get really bad teeth.