My boyfriend tested positive for mono, and he's had it for about 3 weeks. i have no symptoms yet. how can i prevent it? should i see a doctor?

No: Mono often occurs in early childhood as a simple virus that is similar to most others & seldom results in a doctor visit or diagnosis. They are then immune. It is more intense in teens or adults, but takes a good 3-6 weeks before most show signs. Antibiotics would not help anything (virus). Testing before any signs of illness is usually falsely negative. I'd just wait & see, you may be immune.