It's an unual experience for a puppy to have a lead on. Pop a flat collar and lead on in the garden / house and treat / praise him for wearing it. Then build up to walking him on it in home, then slowly progress outside you front door. I found taking my dogs favourite toy out with me worked.
You can socialise him in the mean time by taking him to park, town, vets (for weigh in, just say hi so he has positive experience there), village fetes are now in season, car boots are also great. It's good to go to these places so he meets a range of people (and equipment like buggys, walking sticks) and dogs. It also teaches them that they cants say hi and jump up at everyone so hopefully with training they learn to stay calm round crowds.
When he does start walking on lead try not to over do it, it's more experience and socialising under 1 than long walks as they are growing.
Vet recommend feeds are not necessarily the best, read the ingredients on the side, vets will recommend what they sell as they get profit from sales. Unless there is a veterinary need for him to be on it then I would advise you to do some research on a dog forum or Internet. If he is showing sensitivities it is highly likely highly processed food will make him worse. Like IBS in humans, high processed and high wheat foods make worse reactions.
Just as an example look on internet at the ingredients of wainwrights, and compare to your vet diet and say pedigree. You will be surprised and the less ingredients and chemical names the better.
I had my first giant dog like that and was feeding James well loved as it said sensitive and everything else made him very ill, although he was still sloppy on what I was feeding and barely ate.
After a few years of experience, research and speaking with other giant owners I started feeding natural and never looked back. He wanted to eat, put weight on for first time in 5years, had firm poos only once or twice a day and the size of them were smaller than most spaniels as he was eating so what he needed rather than filler therefore less waste!