Some forms of hepatitis (doctor
speak for liver inflammation) are very contagious, but it depends on what kind of hepatitis your boyfriend has as far as what your specific risk of contacting the disease might be. For some kinds of infectious hepatitis, sharing body fluids or food is a common method of transmitting the disease. This is usually the transient kinds of hepatitis that are commonly transmitted from eating at a restaurant with sub-standard employee hygiene.
The kinds of chronic hepatitis are much more concerning, and sharing a toothbrush is just the start of the prevention that you need to be doing. Hepatitis B and C are very contagious, and are commonly shared through anything that transfers body fluids. Sharing a needle, as in IV drug abuse, or sharing semen at any level is a common method for the transmission of these diseases. There is a vaccine available for Hep B, but Hep C is very contagious and there is no vaccine.
Any hepatitis can ultimately lead to cirrhosis (a serious liver disease) and even liver cancer. You should take steps to prevent this transmission, including speaking with your doctor and AT LEAST using barrier methods of contraception
(condoms) during all sexual activity. Even with this, speak to your doctor so that you fully understand your high risk of acquiring this serious medical condition if you continue to be close to this person.