I would need to see the court order regarding your son's rights but presuming that you are not given any rights in the order you would have to request grandparent visitation rights by bringing a motion in that case and serving mother and father with that motion.
It is very unlikely. We need to know what the father says are the reasons for being mia for 5 years. Your daughters opinion may be asked and considered if a guardian ad litem( lawyer for your daughter) is appointed after court ordered mediation fails. If there is a dispute concerning placement mediation will be ordered first.