Hi Melissa! I know this is tough, but stay positive!!! You're in my thoughts and prayers.
My sis has a brain tumor that sits on her speech center - she's had it (they are guessing) pretty much since she was a toddler. She has very similar symptoms to what your cousin has - except that they impact her speech. She will just sort of "go blank", and a couple of minutes later, she's back with you and, while she's a little loopy (sort of like when you just wake up from a nap), she's perfectly fine. They call them neuro-motor seizures.
She's pretty much fully functional during her seizure, except when it comes to speech - if she's cooking dinner, she continues to cook, etc. The one rule that she and her hubby have is that if she's driving she has to pull over and wait until she's all back to normal - just in case.
She's been treated for her seizures (the symptoms) thru medication literally for as long as I can remember - probably since the first or second grade. And the meds are okay - they have gotten better thru the years, and while they aren't side-effect free, she manages just fine. Mostly they impact her short-term memory so that she has to write everything down so she doesn't forget things - like when to pick up her kids from their friends houses.
Her tumor is malignant, but very slow growing. They didn't even really identify it as a tumor until she was just out of high school and starting college - sort of like your cousin. She had surgery about 15 years ago and they removed what they could of the tumor from her noggin. The speech center sits right in the middle of your brain, so they couldn't get all of it without doing damage. Then they treated her with radiation - I don't think they use chemo much for brain tumors because the drugs tend not to pass thru the blood/brain barrier in the head. Since then, the use the drugs to control the symptoms. They are thinking in another few years that they'll re-do the surgery just to make sure everything is as clean as it can be.
But, the important thing is that sis leads a totally normal life. She graduated college, married, has 3 of the most adorable (and totally healthy) kids you'd ever meet. If in fact your cousin needs surgery, and if his brain geography is correct and the tumor is outside of his brain, they'll probably be able to take most of it with no problems. Plus, if he's been having seizures for several years, it doesn't sound like it's one of those devastatingly fast-growing tumors, which is really good news.
If you want to PM me let me know - I'll be happy to put you or your cousin in touch with my sis, her docs or whomever.
Big hugs!!