Am a solicitor but not a family lawyer. Have however dealt with several court orders, incl in divorce. Quartz is right re definition of "best endeavours". But that's not the whole picture.
You haven't put the full wording of that section of the order, but typically Court orders will say:
The court orders that, upon xyz happening (the conditions to the order), abc will/must happen (the actual enforceable part of the order).
In other words, there'll be a list of conditions (including the one re cubs) which, once fulfilled, will give effect to the order of the court that follows at the end of that whole section (which in this case I suspect is where the court orders you to give your ex access to your son on Fridays).
Important to note is whether the conditions only say "upon the respondent agreeing to use his best endeavours..." or whether they also say something like "AND upon the respondent so using his best endeavours..."? The former statement is fulfilled if your ex simply makes you a promise (whether he keeps it or not) whereas the latter statement says that he MUST keep that promise.
If I have guessed the above contents of the order correctly, then if your ex has fulfilled the condition (see above), then you must comply with the order to give him access or he has the right to enforce and you would be in contempt of court if you did not comply.
If however he has not fulfilled the condition fully (again, see above) then the order does not bite and there is nothing for you to comply with, i.e. You could deny access and he couldn't do anything about it unless he took it back to court to insist that he did fulfil the condition.
Obviously the above is based on pure guess work on what is within your CAO, which I haven't seen. Hope that helps give you a better idea of what that gobbledygook means, but please do talk to your solicitor for actual advice.