well i would give the school a ring. Even though u may be his step mum.. u are with his dad. So it would be best for his dad to ring. Schools are funny like that, they dont like giving out personal information. Ask the school if u and DH could come in and see the social worker. Talk to the social worker about ur concerns and see if they will send some letters home with DSS to his mother stating that he has been absent so many day and its not acceptable iykwim. To me, a 10 y/o not going to school is a big issue. Have u tried ringing his mother and talking to her about this or is she one of them mothers??!? iykwim. Also u could try get an appointment with the principal. Dont tell them when u ring that ur not on DSS file, wait till u are in there. then they cant get rid of u so easy....
HTH..