Cholinergic urticaria

Question:

1/12/2019
A 24 year-old man has episodes of small, "pink" (more than skin colored), flat, itchy, painful, ovoid, uniformly sized spots that usually start on his upper and lower back>chest and flanks, occasionally on his feet or arms or posterior neck starting four years ago. It spared other parts of his body. Initially, the episodes occurred during hotter months and only if he exercised and lasted 5-15 minutes, resolving even if he continued exercising. This occurred 5/7 days but not every time he exercised. If he exercised later in the day the reaction was less severe than if he exercised early in the day. If he had the reaction early in the day and exercised again later in the day, he would not have the reaction. For the past 6 months (especially for the past 1 month) this occurs if he is embarrassed and also with minimal exertion, especially if it is warm. If he moves to a cooler area at the start of the reaction, the symptoms improve quickly. He does not have this reaction if he is taking exams (stressed) or if he takes a hot shower or sits in a Jacuzzi. With these episodes he has no respiratory of GI symptoms. He has been taking cetirizine, but it does not prevent the reactions.

He has had dermatographism for 10 years. He has been a research technician in Catalina 4/5 and at USC 1/5 days a week for 6 months. He cleans fish tanks ("touch" tanks that people visit to touch fish and fresh water tanks), is exposed to some local marine species and does not wear gloves. He was in SE Asia (Borneo, Malaysia, Thailand and the Philippines) for 4 months scuba diving and lounging on the beach from 11/01/17-02/01/18, home in Laguna Beach for 5 months and in Catalina in an apartment for the past 6 months.

He has found ticks on his body (never attached) and has not visited the east coast.

His father has Crohn's disease and Legg-Perthes disease. He has red hair, blue eyes and fair skin. He has mild dermatographism. Otherwise there are no other notable physical findings.

Could this be cholinergic urticaria? Would it be unusual in cholinergic urticaria to have no reaction if taking a hot shower or sitting in a Jacuzzi? Could any of his exposures be relevant? Could it be a mast cell disorder? What is your differential diagnosis? What lab studies would you recommend (I requested some). For now, I recommended he take fexofenadine or loratidine and an H2 blocker twice daily.