I almost hate answer questions like this online bc I hate to make you paranoid for nothing. . .

But if it were my child. . .
I would be concerned enough to call the pedi and ask for an extra appointment just to check it out unless you're going soon for another well baby check up. Why? Because according to every developmental milestone marker guideline out there, babies coo between 0 and 2 mos old. To not really coo by 11 weeks is late.
HOWEVER, (and notice how I put that in big, bold, capital letters?) just because he's not cooing doesn't mean something
is wrong. Babies have not read the textbooks and have not been informed of the charts and so consequently they just don't always follow it.

It's really when he misses several developmental markers in a row that
you have to start being concerned. All that to say, yes, check it out. It's definitely on the much later end of when he should be doing it but your pedi will look at a whole bunch of stuff and chances are it's all fine.
