"Subject to" for sellers?

[Christian Bryce]

I am considering purchasing my first deal from my father as he is about to retire. I am weighing my options of either assuming the current mortgage or doing a 'subject to deal'. The seller, being my dad I am concerned with leaving the mortgage in his name and how it will affect his ability to purchase his next home for retirement. My question is: Will he be able to prove that I now am making the mortgage payment for this house or will it count toward his debt to income when he applies for his next mortgage? Thanks!