The probability of a child to develop Bipolar Disorder if one parent has the illness is estimated at around 30% and keep in mind that this is a probability that reflects a trend in the wider population - so it doesn't really apply to any one individual.
The typical age of onset for males is late teens to early 20s so it's a great sign if he's not showing any signs so far. As a person with BPD I'm sure you're aware of the various risk factors and it is very important he be mindful of these through his current age period. Do you have any additional questions? I will await for your response to what I have written so far.
Yes you are right drugs is the most important risk factor to avoid (especially Marijuana). Drinking occasionally is fine as long as he tries to avoid binge drinking. He just generally needs to look after his health and try and make sure he keeps a stable sleep pattern. Intense stress & anxiety can contribute to the deterioration in mental health so he should also try to manage his anxiety in a healthy way.
It is generally accepted that illnesses like Bipolar Disorder are caused by a combination of genetic and environmental factors so none of the factors listed above will 'cause' Bipolar on their own (sorry if I'm telling you something you already know).
I hope this is of some help. Please let me know if you would like more information on this matter or if you have further questions.