Which ADHD diet are you thinking of? To my knowledge, the one most famous and most supported by studies is the Feingold Program, Stage 2, and it's the one I know the most about, so I'm going to write this post with that in mind. (The Feingold Association recommends against skipping to stage 2, but I recall a few anecdotes about that working out well enough for people who couldn't do stage 1.)
What kind of schedule does his dad have him? You are supposed to be on the program for a few weeks before deciding whether or not it's working, but I've heard stories of kids showing signs of improvement in just a few days. So if his dad has him just on the weekends or something, it might be worth a shot. Also, if your kid really starts to believe in the diet, he might avoid questionable foods regardless of what his dad offers him.
Contrary to common belief, the Feingold program does not mean cutting out sweets and junk food. It's also not about natural versus unnatural. It gets rid of these:
--These preservatives: TBHQ, BHA, BHT
--Salicylates, a category of chemical that occurs NATURALLY in some fruits and veggies (avoiding these is Stage 1)
--The materials they send you if you sign up to the program also have some resources for avoiding sodium benzoate, casein, and gluten
Natural sweeteners (sugar, vanilla, xylitol) are not forbidden, but you probably want to exercise moderation for other reasons....
You can still have all sorts of tasty foods on the program. The problem is you can't just eat anything anymore. For example, if your kid wants a cupcake, you can easily make cupcakes without the forbidden ingredients, but if one of his classmates brings in cupcakes for a class birthday, the chances that those won't have the forbidden ingredients are pretty slim. Also, his favorite cereal or something might have the bad stuff, and he might miss having that beloved food. He might also have a loyalty to certain brand names even if there's a nearly identical safe food in another brand.
The hard part about determining which foods are safe is that preservatives don't always have to be listed on the label in some situations. (Plus, if you're avoiding salicylates, that's often in "natural flavoring.") So if you want to be able to buy anything processed enough to have an ingredients label (including staples like milk and cheese!), you'll either have to buy the booklets from the Feingold Association or call the manufacturers yourself. If you're pressed for money, you might try the no-processed-foods tactic for a while to see if your child shows improvement, then if it works, buy the booklets later to make your life easier. I think it's like $80.
Also, it's not just a diet. You also need to avoid cosmetics, medicines, soaps, etc. Anything that goes in the mouth is probably higher priority (toothpaste, dental floss coating, lip balm, mouth wash, vitamins), but you need to worry about stuff going through the skin too. I had a lot of trouble finding OTC medication that didn't have food coloring.
For the record, I hear very little about the Feingold Program or other ADHD-improving diet fixing attention problems. The focus is usually on behavior problems caused by hyperactivity or impulsivity. "Inattentiveness" is sometimes listed among a big list of symptoms the program can lessen or fix, but I'm starting to think those lists might refer to inattentiveness as a manifestation of hyperactivity/impulsivity (e.g. can't concentrate on a book because you're too busy bouncing off the walls) rather than the attention problems faced by people with "primarily inattentive-type" ADHD. I see slightly more info about it fixing sensory issues though. I heard a few stories about kids becoming less picky eaters once they started the program.