When someone is sweating a lot when it is not hot or they are not working out, and they are going to pee a lot, that would indicate a possible kidney infection.
Other reasons for peeing a lot can sometimes indicate diabetes.
Getting dehydrated and also having a urine infection can cause headaches.
He should drink plenty of water throughout the day. 2 litres (5 large glasses) of water is usually recommended unless advised by the doctor or urologist. If your boyfriend sweats a lot (for whatever reason) he would need to drink more.
When he is like this, it is a good idea to get him to check his pee. If it is cloudy, smells strong, or dark in colour which could mean there is blood in the pee, he would be wise to make an appointment to see his doctor to examine his urine (pee) and check him out. When someone is dehydrated (does not drink enough) the pee can be a dark colour. Normal pee should be a light straw colour. Eating beetroot and drinking coloured drinks can also cause the urine to be a different colour.
If he is still having a problem and is feeling week, he should make an appointment to see his doctor to run some tests to see if there is an underlying cause that needs treating.
Best wishes.
Besides water it's good to have those packs of Emer gen C. Vitamins and electrolytes.