I.G.
The most important thing: RELAX. Assuming that he's a normal 6 months old, he probably won't starve...
It pretty normal for babies to have a variable appetite at that age, he might be teething, or coming down with a virus, that will affect his appetite.
If it's hot where you are, he may go for short nursing sessions, to get more watery foremilk because he's more thirsty than hungry.
If you have started to introduce solids, that may also play a role.
Just try to relax and keep on offering...some extra cuddle and skin to skin time will also help, especially if he's cranky.
Last but not least, you mention he's only had 2 "decent" feedings... if he feeds even for just a very short time, he might be getting more than you think. Babies can be very efficient nursers at this age!
I bet in a day or two he will make up for what he's missed.
Good luck.