I was suffering from the same thing last week. My DD is 13 months, had a cold, then a start of an ear infection
DD's milk intake was just taking just under a pint but she would drink lots of cooled boiled water during the day.
We had a few bad food days, where my DD would only take fromage frais. One day DD had 4 and that was it!
Food intake is still very much up and down. Yesterday she ate alot, today, quite a big reduction. So I sympathise.
Is your 11 month old teething by any chance? I know that when my DD is teething her appetite does reduce significantly.
If he is fine in himself during the day, then they pick and choose when and what they want to eat. Try not to get stressed or make a big deal about it. Babies at this age know that if they can get a reaction or that they know that they 'push your buttons' they will continue to do it. We always try and make eating as 'pleasant' experience as possible as well as shared. If DD doesn't want the food, we don't force the issue.
Alternatively, if in doubt speak to your GP or Health Visitor.