Salamu alaykum Suppose someone fasted but broke his fasts without a valid excuse and was slacking in his prayers (not consistent) in his ignorant days and he repeated to Allah and was extremely regretful years later, what should he do with his fasts and prayers? Should he make them up for each day?

Ustadh AbdulAziz al-Haqqan

If he knows the days of fasting then yes he does them, if not he estimates to the best of his ability. As for the alah repent and increase your voluntary prayers.