Thanks for using Health Care Magic. Read your query. The symptoms mentioned by you suggests reactions to the mouth wash. Yes , he can continue with the normal saline mouth gargling. If needed he can take a anti allergic tablet like loratidine which will help reduce the allergic reaction caused by the mouthwash.(call your dentist office before taking any medicine). For the mouth ulcers , apply mucopain gel on the lesion two or three times a day till it subsides.Reduce spicy food and avoid stress . For tha abscess and dead tooth , he has to undergo a root canal treatment after having a radiograph done. Pain can be reduced by using ibuprofen tab (if he is not allergic to any medication),also a dose if antibiotic may be required with the doctors prescription. Hope this was useful. Thanks and regards.