Hello georgeriley
I am William a Child Maintenance Option's consultant. As long as you do have parental responsibility, once you have established which school or college your son attends, you are able to contact them for written confirmation that your son is attending. If you find out that your son is no longer in education then you will also be able to ask for written confirmation that he has left.
For information on parental responsibility and the parental rights this gives you, you can find information on the Government website Gov.uk using the following link, https://www.gov.uk/parental-rights-responsibilities.
In your post you mentioned that you pay child maintenance through the Child Support Agency (CSA). The CSA rules say that child maintenance must be paid by the paying parent to the receiving parent until the child turns 16 or until they reach 20 if they are in full-time education. The definition of full-time education is more than 12 hours a week of study, on a course up to and including A level standard, or for as long as Child Benefit is being paid. It is also worth noting that when a child leaves full-time education in the summer, Child Benefit generally continues until the first week of September.
To find out more about how Child Maintenance Options can help, or for information on all the options available to put in place a child maintenance arrangement, including online tools and forms to help with family-based arrangements, you can visit our website at http://cmoptions.org. If you would prefer a friendly and confidential chat, please call us on 0800 988 0988 (free from a landline).
We also have a sorting out separation web-app, it aims to make it much easier for separated parents to find the support they need, when and where they need it, and encourages parents to collaborate on a range of issues. The link is: http://www.dad.info/divorce-and-separation/sorting-out-separation
Hope this helps.