Hi Liffy
As with any numerically popular breed, not all Cockers have good temperaments as sadly not all pups are bred by reputable breeders who put sound temperaments at the top of their list of priorities. However, this is a long way from saying that all solid Cockers are aggressive (as some ill-informed people do claim). There is a specific condition known as Cocker Rage Syndrome which is mainly seen in solids but it must be stressed that the condition is rare. Have a look at
http://www.cockerspanielrage.org.uk/ for more information on this subject. Your Cocker is still classed as a solid even if he has small white markings under his chin & chest (often seen in solids with some particolour breeding in their background) but please do not worry about his temperament - many, many solids have lovely temperaments & it sounds like your pup is one of them :-)
Jane