I am assuming you know his carbohydrate to insulin ratio, meaning for every 10-17 carbohydrates a person eats then one unit of insulin is given. I give a range because every person is a little different in how many units of insulin they give for a certain number of carbohydrates. I usually take 1 unit for every 12 grams of carbs--but that is just my body.
Next, knowing the amount of carbohydrates in certain foods will help too. You can find many books online or at the library or book store that name many foods and how many grams of carbohydrates are in those foods. There are other factors involved too--how much fat that is in a food can make a difference too. French fries are digested differently than say a baked potato.
Now, that may sound scary, but it doesn't have to be difficult. Try to make sure you are balancing carbohydrates and proteins with every meal. Make sure he gets a well rounded meal that is in good portion control. Also if he eats breakfast, snack, lunch, snack, dinner, snack--smaller meals all day may be very helpful as well.
I looked online and it was a little difficult to obtain the information you are looking for. I would try to narrow down the search a little bit. Try and find out the exact areas you plan to eat at and narrow it down to maybe a smaller number of restaurants. I think you might have better luck that way. Maybe through more search(or someone else on the site might know a specific book for what you are looking for) you can find a website or restaurant guide with all the information you are seeking.
Excellent advice from Forum-vol-cmb. I find that when traveling and eating out, I do best going to restaurants that serve a big variety of foods so that SOMEWHERE on the menu I can find foods that have fairly easy-to-figure carb counts. For instance, for a child, obviously a sandwich on regular bread rather than a large sub roll is going to be fairly easy to figure out. The other thing is that when eating out a lot, it probably behooves you to have him test his glucose levels more often than he does at home. Both to catch hidden carbs in foods and also because his activity levels may be very different from the norm. If he tests a couple of hours after the meal, he can correct if your estimates were off.
Some restaurants, particularly chain restaurants, may have carb counts available, but I think you will just have to ask in each restaurant if a carb count is available.