
Hi Debi
Glad you have made progress with Ralph's behaviour :-)
I'm not the best person to advise on food as I feed raw but I can say that some dogs are just food obsessed...I have one right here. However, if Ralph's weight is ok (and it does sound to be roughly what I'd expect for a male of his age but it's impossible to say for sure without seeing him) the you're feeding him enough. I'm sure you know it's always better to keep the giant breeds on the lean side :-) For what it's worth, I see no problem with changing him to a lower protein food at this stage if you want to.
Do you give him recreational bones or anything like that to munch on? If not, it might be an idea to try him with them although I appreciate that not everyone is happy with the idea of giving raw bones.
With regards to his behaviour when out and about...well, I have good news and bad news. The bad news is, he has hit the dreaded teenage stage. The good news is, it only lasts about 12 months

:-D
Sorry, I don't recall if you use a head collar but if not, do consider getting one. The
Black Dog one is very good but if you do a search for head collars, you'll see lots of recommendations for different brands.
At your classes, have you been taught a 'watch me' command? This is great for getting them to focus back on you and away from whatever they're fixating on. If you haven't been taught it, take a piece of high value food and get Ralph's attention with it, then take it and hold it up to your eyes, when Ralph is looking at your eyes, give him the food. Repeat several times, adding in the 'watch me' command (nice excited sing songy voice). Eventually, the second you say 'watch me' he should look directly to your eyes. Practice when out and about when he isn't distracted by people/dogs etc. Once he's responding to the 'watch me' command, then you can put him in a sit and make him wait for the distraction to pass before continuing with your walk. It's long and frustrating process to begin with and ou may find that you don't get awfully far on walks to begin with, but pererverance is the key :-)
As for constantly hunting for food on the pavement...well, if anyone has the answer to that one I'd like to know!! All I can say is that my trash hound isn't as bad as he used to be but if he detects food on the ground (and I swear he can sniff out kebab remains from about half a mile away!) then he'll pretty much stop at nothing to get to it. I generally pick the offending item up in a poo bag and stick it in my pocket to bin when I get home...while it's in my pocket I can pretty much guarantee that I have his undivided attention so every cloud has a silver lining ;-)
I would continue walking Ralph without the children in tow as he really is at the stage where he needs your undivided attention. I know it's frustrating for you but the more work you put in now, the faster it will start to pay off. You've achieved such a lot with him already so do give yourself a well deserved pat on the back :-)