Your offer of financial assistance is very generous; however, mortgage loans do not have cosigners like automobile financing does. In a mortgage loan the other person on the loan is called a coborrower, which may not be the type of assistance you are looking to provide.
In a cosigner agreement the lender looks to the primary borrower first. The cosigner is only billed if the primary borrower defaults (does not pay) on the loan. Especially when the cosigner is a parent, the lender will use the cosigner’s credit rating to authorize the amount borrowed.
In a mortgage loan the coborrower is just as liable for the first payment as the other borrower is. The credit records of both borrowers are used when determining the type of mortgage offered and the interest rate. A default on a mortgage loan equally affects both coborrowers.
Instead of putting yourself up as a coborrower, perhaps you could provide your son with money for a larger down payment on a home. Because the lenders look not only at credit rating but also at the down payment amount, a larger down payment may allow your son to get that lower rate mortgage.
As for your son not having credit, have him request his credit report to see if he does have credit. Help him accumulate a good credit history by applying for easy-to-pay-back loans and credit accounts. Remember to stress that he must make every payment on time. In the meantime, you can help your son out by assisting him in looking for a home that is within his financial means.