The witching hour. All of my kids have gotten fussy later in the day like that, though to varying degrees. E is more fussy now than M. We've solved it so far by putting them to bed early - the girls go down around 6 or 6:30pm (they are usually up twice at night to eat, and up for the day around 7am, sometimes 8am). The boys used to go down really early like that, too; now they go to bed at 7:15pm.
The other thing is that babies often tend to cluster feed in those hours getting ready for the long night. Tonight, M took two 4oz EBM bottles in less than an hour, and then wanted more an hour later. Not sure how much she took that time as I had sat down to pump (and still am). So even if you think that he shouldn't be hungry, he might be looking for more to eat (and sometimes we have to coerce the girls to take more but judging from the amount they then take, it was what they wanted).
You may have tried all this already, just wanted to mention what has helped reduce the evening fussies in our house for our kids!
Single mom to
DS ("twice exceptional") - September 2002
DS - February 2006
DD - July 2009
DD - July 2009