Social Services would be far more likely to be concerned about a parent with a POSSIBLE bipolar disorder (who did absolutely nothing about it) than with a parent with a DEFINITE bipolar disorder (who had sought help to get it under control).
The information on the NHS website might help you:
http://www.nhs.uk/conditions/bipolar-disorder/Pages/Introduction.aspx
and you can find a local support group through Bipolar UK:
http://www.bipolaruk.org.uk/
However the first step HAS to be be to talk to your GP about your concerns.
While I'm not trying to minimise the impact that Bipolar Disorder can have upon people's lives, it's worth remembering that many people with BD manage to cope with it remarkably well. For example, Stephen Fry and Catherine Zeta-Jones both have BD but they seem to have it under control.
If you've got the courage to post here then you've almost certainly got the strength of character to seek professional help. In you're still not sure, just ask yourself what is best for your son. Do you want him to grow up confused and insecure through living with a mother who exhibits erratic mood swings or would you rather that he has the sense of security that is only available to children when there is consistency in their lives? Seems like a no-brainer to me - seek help!