If your dog has allergies, it is likely his skin is dry and irritated. Patchy red skin can be painful and can hurt. Before you bathe your dog, you will want to be well prepared with the right kind of dog wash or shampoo from your veterinarian or groomer or be prepared to make your own shampoo or wash to treat the symptoms of your dog's allergies. Warm water will be quite uncomfortable for a dog with allergies so be sure the water is not too hot and you are aware of how well your dog is handling the bath.
Remember if your dog has allergies, he is probably itching and scratching quite often. His skin is probably dry and flaky causing his fur to also be dry and coarse. If your dog has allergies and irritated skin, he's not going to want to be wet for very long, and his skin may be so sensitive to the touch he may not want to bathe. Focus your attention on making his skin feel better, so his baths are soothing rather than irritating.