No. The above posts are very misleading.
What you want to do was possible up to 2008. Since then equity release has obviously tightened up a lot. Banks will provide equity release for very specific purposes, such as home improvements or paying college fees for your children. For any material amount they will seek evidence of the spend. Buying a 2nd home, holiday home, or an investment property is not a purpose that banks will provide an equity release. (By the way, they won't provide it to buy a Maserati either, not in the last 12 years anyhow).
Your only options are either a mortgage for your son (I'm assuming that's not a runner), or a BTL mortgage, secured against the new property. You will need 30% deposit.