LB, I have had 2 cavaliers, who seemed extremely fussy. They left pretty much all food I offered, but I just made sure there were always complete biscuits available at all times. I never actually really saw them eat it- but they were both healthy, energetic and active- and obviously didn't starve themselves.
With more than one dog, now, they all eat (even the cavalier) as soon as it is down, just due to competition. So- I would just make sure food is always available and leave them to it. Unless they are clearly starving or unwell. I guess he has had his teeth checked etc?