Can you get good quality puppy food, which will have the right ratio of nutrients, and if it is dry food, grind it into powder and mix with water?

Please don't leave him in the shower cubicle. Apart from the risk of him slipping and hurting himself, he will be alone and afraid. That's why he is crying. He needs you to be with him. You do not want this dog to develop any more insecurity, he has missed out on vital lessons from his mother and littermates already.

If you are concerned about catching viruses at the vet, carry him in and don't put him on the floor. Carry a disinfectant spray and spray the vet table yourself, if the vet doesn't do it first. Make sure the vet washes his hands thoroughly before handling the puppy. If they have any objections to any of these reasonable precautions, find a different vet.

 

Second the puppy food. If it's dry, you should soak it in warm water and then it should mush easily into a paste. Put the food in something flat, like a plate, not a bowl, to allow him easy access. Place him in front of it and he should hopefully do the rest. There are ways to entice them to eat, but I wouldn't bother with that unless he is reluctant to try the new food.

Are you able to weigh him? He should be gaining weight every day, about 10-15% of his body weight. Don't worry too much about the math if you find it difficult to understand (it's definitely not my strong suit!) as long as he's gaining some weight every day or so and definitely not losing any weight.

Puppies can't regulate their own temperature effectively, so the cold tiles or concrete of the shower isn't the best place for him. If you can get newspaper or some old towels, these make good bedding materials. You can wrap a soft toy/teddy in a shirt that you have worn so it has your smell on it for him to snuggle up with when you can't be with him.

Worming him is a great idea, I'm not sure how it works where you are but a lot of vets will sell worming solution over the counter without an appointment. Other than that I second the advice JoanneF has given you on making a vet visit as safe as possible. He'll need to see a vet for his vaccines at 8 weeks old, so the vet isn't something that can be avoided.

It sounds like you are doing the very best you can in a difficult situation, thank you for looking out for him and well done for reaching out for advice when you needed it. Sometimes that's the hardest part.