You need to be sure that it is the same CerAxon that you have been giving as a tablet. I can only find tablets from their own website.
IF indeed it is than you can give the same dose of 500mg. IF it is 1000mg per "1" (or whatever it says) cc or ml than you would give 1/2 of that. IF you are confused then let me know the exact dose and I can help you further. It will say something like:
1000mg per 2ml.