I am sorry to hear that your dog is going through this problem that sounds like it is fairly uncomfortable for him. Everything you have done so far is absolutely correct, but unfortunately if it is a deep wound that then became contaminated with maggots it is likely that there is a systemic infection now. That would account for all of the swelling that you are now seeing as there is probably a large amount of cellulitis.
The best thing that can be done at this point is to have your vet examine him and prescribe oral antibiotics. They may even need to explore the wound and debride any dead or unviable tissue that may be present. Feet and paws, when infected, often need longer term antibiotics to heal. They may even want to prescribe something for pain.
Good luck with him and I hope everything heals well and without problems. Please let me know if you have any other questions.
The fish mox is not "like amoxicillin but better", because it is just amoxicillin that is marketed for fish. My concerns are that, while amoxicillin may work for some puncture wounds, it may not be the appropriate antibiotic choice in this case. If you have already been using it for 6 days (at an appropriate dosage), you should be seeing some improvement. Reasons that you are not may be that the bacteria involved is not susceptible to the amoxi or that there may still be too much diseased tissue that needs to be removed or even perhaps, there may still be a foreign body present if that is what caused the puncture in the first place.
I still believe it is in your dog's best interest to have an exam and probably at least a culture of the wound to determine what bacteria are involved and what antibiotic it would be susceptible to. You don't want this infection to get out of control and become a systemic problem as then it can lead to other problems besides just the foot.