It is possible as there is a strong genetic link, but I have never seen a study giving the percentage chances. And it is a few years before that should become an issue. I would spend my time learning about ADHD so that you can better understand what your boyfriend has been going through.
One site that you both might enjoy is this adult site for AD/HD - http://jeffsaddmind.com/for-first-time-visitors
The more the both of you learn about ADHD, the stronger your relationship will be and the better it will be for your baby. If you have any more questions, feel free to post. Best wishes!
I don't think it is a guaranteed thing however my friends husband is ADHD and so is their son but not the daughter