Clearly, you and your son can be co-applicant on the housing loan. However, your son is entitled to deduct 100% of the mortgage interest on his tax return subject to the fact that he has paid 100% of the mortgage payment.
But if you paid 50% of the mortgage for example, your son cannot deduct mortgage interest related to your payments.
The IRS spells it out very clearly in Publication 936, and states that a taxpayer may not deduct mortgage interest if he has not paid the mortgage payments. In other words, your son cannot claim the mortgage interest on the portion of mortgage payments paid by you!
But, if your son makes all the mortgage payments, then in my opinion he is entitled to deduct the entire mortgage interest and property taxes on his tax return.
If your son were to claim 100% of the mortgage interest deduction on his tax return, and he were subject to an IRS audit, he would need to prove that the mortgage payments were paid from your son's bank account only. So, to protect yourself, it is best to ensure that your son makes all the mortgage payments from his bank account using his funds only!