You can probably use the sieve sub-address extension, most e-mail providers support that. After the username part of your e-mail address add a + (plus) and any other string, like your sons name. Your e-mail provider will just deliver it to you, but Bunq will hopefully accept it as a unique address.
If for example your e-mail is jasper(at)example.com, you can use jasper+myson(at)example.com
