If you break a lease and you know your landlord is moving back into the house, can he hold you responsible for the duration of the lease?

Your obligation to pay rent ends when the place is re-rented or in this case if the landlord moves in and decides not to rent the place.  The landlord has a duty to mitigate (minimize) damages by either making reasonable efforts to find another tenant or in this case by moving back in and not renting the place.  The landlord cannot move back in and still claim that you owe rent as this would be a failure to mitigate damages on the part of the landlord.  If the landlord fails to mitigate damages, the landlord's damages (the amount claimed that you owe) is reduced accordingly.

If the landlord re-rents the place, the landlord has to make reasonable efforts to find another tenant.  Reasonable efforts would depend on what other landlords in the area are doing to attract tenants; for example, posting a sign on the property advertising the vacancy, advertising the rental in the newspaper or a local rental guide or online, etc.  If the landlord fails to mitigate damages, the landlord's damages are reduced accordingly.  The landlord cannot allow the place to remain vacant for the balance of the term of your lease without making reasonable efforts to find another tenant.  If the landlord does that, the landlord has failed to mitigate damages and  the landlord's damages will be reduced accordingly.  If the landlord re-rented the place and charged the new tenant less rent than you were paying, you would remain liable for the difference in rent for the balance of the term of your lease.  However, the landlord needs a valid reason such as market conditions to justify the lower rent; otherwise, the landlord has failed to mitigate damages and the landlord's damages will be reduced accordingly.