Answered in your other post.
https://www.irs.gov/taxtopics/tc505
Qualified mortgage interest includes interest and points you pay on a loan secured by your main home or a second home. Your main home is where you live most of the time, such as a house, cooperative apartment, condominium, mobile home, house trailer, or houseboat. It must have sleeping, cooking, and toilet facilities.
Note that it's interest and points YOU pay on a loan secured by YOUR home.
In my opinion, your boyfriend has an ownership interest, but the your son's home is not a qualified home for your boyfriend. If your boyfriend is helping with the mortgage payments, he cannot deduct the interest included in those payments.
It's either legal or equitable ownership that is the test which means the boyfriend needs to show he has the benefits and burdens of ownership. He may have one as a cosigner but not the other.
Probably not. In order to claim the home mortgage interest deduction he would need to live in the home either as his first or second home and also pay the payments. If your son is making the payments and living in the home, the interest is his to deduct.
As for "Write off" mortgage interest is only deductible as an itemized expense. Itemized expenses include mortgage interest, gambling losses up to winnings, charitable contributions, state and local taxes up to $10,000, medical expenses in excess of 7.5% of your AGI and casualty and losses in excess of 10% of you AGI with the first $100 not counting towards the loss. Your health insurance and all medical expenses are only deductible for the amount that is over 7.5% of your AGI. This means if your AGI is $50,000, then the amount that is over $3,750 is deductible.
Then your total itemized expenses would need to be greater than your standard deduction below in order to benefit from your expenses.
The 2023 Standard Deductions are as follows:
Blind or over 65 and MFJ or MFS add $1,500
Single or HOH if blind or over 65 add $1,850
In order to claim the home mortgage interest deduction he would need to live in the home either as his first or second home and also pay the payments.
He doesn't have to live in the home but he does have to make the payments in order to deduct the interest and he does need to have legal or equitable ownership.
Generally, home mortgage interest is any interest you pay on a loan secured by your home (main home or a second home). The loan may be a mortgage to buy your home, or a second mortgage......The loan must be secured by the taxpayer’s main home or second home (qualified residence), and meet other requirements. Pub 936