The prior months interest payments on the home converted to a rental and half of the interest payments on the new home are deductible on schedule A for your son. The amount of the mortgage limit depends on the average balances. Since the rental home mortgage was originated in 2017, it is subject to the $1,000,000 limit. The actual limit in your son's case comes down to the Maximum of (Minimum of (Average Rental Home Balance,$1,000,000) or Minimum of (Average New Home Balance,$750,000)).
Technically, the averages are computed over 12 months with $0 entered for the inactive months; October-December for the rental. Turbo Tax doesn't do it this way but will compute the averages for you using a simplified method approved by the IRS that is usually in the taxpayer's favor.
