Hi, i need some advice please

wont go into all the background, but we (Dh and I) believe that DSS is not gong to school much atm as his Mum isnt taking him...
Last year he was absent over 30 days and i know that last week he didnt go to school until 10am one morning as she slept in and he only went one other day (he told us).

DSS is 10 and not really bothered by it (who would be, he gets to play all day) but we are concerned as he will fall behind.

We have him every 2nd weekend, but would be prepared to have him all the time (especially at the moment while she cant seem to get her act together) so that he is being properly cared for. But i dont think that she will give him up - she needs him to do things for her (but thats another vent that doesnt need to be in here ) and she would get less attention (she uses him as a poor me) and money...

We dont have any contact with the school, for no other reason than she changed his school and then enrolled him without listing our details.

Anyone got any ideas???
Where should we start? by calling the school?

thanks