No, adjusting the dose does not necessarily mean remicade is not working. My understanding is that very few kids manage on an 8 week cycle.
Is this the first time his levels were checked? Were antibodies present? And why were levels checked?
My son was started at 8 weeks but, at his first 8 week infusion, his levels were tested before the infusion. Levels were low and he was moved to 6 weeks. He's been steady at the same dose, at 6 weeks for approx. 5 years now. His levels were tested last year and all was still good. His remicade levels were actually a fair bit higher than the guideline (unfortunately, I forget the exact numbers now) and I did ask his GI if we could extend out to 7 or 8 weeks. His GI said he was happy with those levels, the guideline was just that, a 'guide' and the higher level gave a 'cushion' in case my son was late with an infusion. Higher levels do not increase risks.
Better to increase frequency and/or dose than run the risk of too low levels. Even if he remains in remission with the low levels for a time period, the risk will be that your son will develop antibodies due to the low remicade levels. Once this happens, then it is quite likely your son will not be able to continue with remicade due to an allergic reaction.
If your son had antibodies present, it could be that the antibodies are offsetting the remicade and there would be the risk of an allergic reaction. However, if this was the case, your GI would probably have mentioned this and possibly have added steroids to the infusion or discussed adding methotrexate to reduce the risk of antibodies forming.
Now, why were levels checked? Routine? Or because he was having symptoms? If no symptoms, it may just mean he's burning through remicade (metabolizing) quickly. Some people just do (my son did/does).
If they were checked because of symptoms, it still doesn't mean that remicade isn't working, sometimes inflammation can burn through the remicade more quickly and, for some people, it takes a higher dose (amount and/or frequency) to maintain remission.
Do you have any follow-up coming up with your son's GI?