Does my boyfriend have legal rights to sue my parents after losing belongings in garage fire?

If you parents' garage caught fire as a result of their negligence then your bf may have a good cause of action to obtain the fair market  value of his intruments.  Negligence is defined as a breach of a duty of case that proximately results in damages.  If you parents were aware of the instruments and permitted your bf to keep them in their garage, then they assumed a duty to keep the house in a safe condition to prevent the instuments from being damaged.  if the fire happened as a direct result of some act or ommission of your parents, then he may properly sue them.  I suggest that your parents talk to their insurance company to get your bf compensation for the items to avoid a lawsuit.