Some people cannot tolerate taking stimulant medication some days and not others. It’s a drastic yo-yo of the way the brain is functioning, including emotions.
I think you’re well within your rights to let him know that you can’t continue in the relationship the way it is because of the volatility and lack of consistency.
Another consideration is that the Vyvanse may not be the right ADHD medication for him. There are two types of stimulants: amphetamine (Adderall, Vyvanse, Evekeo…) and methylphenidate (Ritalin, Concerta, Quillivant…). Almost everyone does well on one type or the other, but not both. Learn more details here:
ADDitude Community Moderator, Parenting ADHD Trainer & Author, Mom to teen w/ ADHD, LDs, and autism