If your parents are dining alone, then your mother could simply pull the crackers away then he'd be hungry for the meal. If need be, she can explain to others,
and staff, why she's doing this. When dining with others and he is more apt to fill up on crackers than finish his meal, I think a good start would be asking the dining server for small portions. This is not a "strange" request as I assure you many people are requesting smaller portions as well. Sometimes we can be overwhelmed by so much food on one plate and this can discourage eating the meal at all. But small portions help some people take a few bites.
One other strategy might be to assure he's getting protein, fruits and vegetables throughout the day so if he does not consume these foods at dinner time, your mother can be sure he's gotten some good nutritious foods that day. This can be done by encouraging the consumption of these foods at breakfast and lunch. Also, she might want to stock their kitchen with healthy snacks such as: fresh fruits, vegetables (carrot & celery sticks) and good protein sources such as yogurt and string cheese. Keeping these foods easily accessible will promote snacking on healthier foods. But the important note is that your father is filling up on something and is not going hungry.