Suggest Treatment For Rashes And Dry Heaving In An Elderly Person

my father is 88 years old. He is dry heaving with no solids coming up and little fluid. he has had a rash this week ( Browntail moths seem to be number one on the quessing list) and Drs. prescribed a topical which he has been applying for the last four days. He ate a full lunch but that appears to be staying in place. He is a wood worker and has been in the shop this afternoon. It s warm...he had no doors or windows open. He drinks very little water and has only had fluid with his lunch today. I am guessing he may be dehydrated and had him drink a few swallows of water which he handled well. Any suggestions? YYYY@YYYY