I would talk to him. My husband's best friend was dating and engaged to a horrible person, she had good work ethic but she was a slut and had this friend wrapped around her figure so tight he did not notice that she was sleeping around and even dating another guy while they were engaged. It finally took us sitting him down one on one, otherwise it is unfair to have two against one. I know my hubby really just pointed out all the bad but I talked more as concern so it would not feel so much like an attack. Finally he broke off the engagement, a few months before the wedding because he actually caught her cheating.
If a son really respects what mom says he will reallly think about the relationship, but if he does not know why you are concerned then he won't back away. I know in the above case the parents were trying to be supportive, did not like this girl too much but did not say much but after the wedding was called off our friend still feels hurt that his parents did not say something. He may or may not have listened to them at the time but as a parent you can at least express your concerns out of care as well as still be nice to the girlfriend.
Unfortunately I have another friend, who is not a really close friend but close enough that I spoke up with concerns about who he was dating. She sounds a lot a like who your son is dating... very lazy, is even at work hanging out (I also work with him), has had more jobs then you count on your hands in the past 4 years but has not kept them for more then a week, she is friendly and talkative but tells you her whole life story so you feel sorry for her (she even asks if you can help her out and if you have ever gone through job hunting and so on)... anyway this friend pretty much did not think twice about what I said he even said well my parents are ok with her and even her parents like me (of course her parents like you because she can not live on her own and you get her out of the house). Now they are struggling and they were living at his' parent's house till they kicked them out and now he asks why... well it ends up his parents think this girl is no good for their son but kept their mouth shut, he said that they told him that BEFORE they got married he would have back off. Once again he said he would have back off, who knows what really would have happened.
You should express your concerns, but also let him know that if does continue in the relationship you will be there for him no matter what. He sounds like a good guy and besides his girlfriend choice has a good head on his shoulders so hopefully he can see that you only want what is best for him.
Oh I just remembered something from my dating years... I was going out with a guy for 6 months and we were celebrating my birthday at his parent's, they got me socks for my birthday, I was shocked but smiled and said thanks. After we left my boyfriend apologized for the gift, he then said his parents were not huge fans of me. I asked why and he said they said I was not "good looking" enough to run in their circles... I laughed, I was teenager, had some acne (still do to this day) and at the time wore glasses (I now wear contacts most of the time) but other then that I am ease on the eyes, a size 0 (which now I am nowhere near now), friendly, smart, nice, the list goes on. This was a rich family, I knew that from the start, but never thought looks would play so much into their thoughts as to liking a girlfriend. After two more weeks we finally broke up, his mother's words just meant more (granted we had only gone out for a little over half a year)... and all of that was brought on by socks, even if they came from whatever high end place they shopped, for a gift. So give socks as gits!
Does the girlfriend talk about the gifts or money a lot when around you and the family. If so mention to your son that she talks about that a lot and she seems to care a lot about those... if he engages in that conversation maybe suggest he spend less on her for a few months and see if their relationship is still as happy/strong.
In the family or extended family is there someone he is really close to, like a cousin or uncle, brother or sister, or maybe a close friend. See if one of them will talk to him (of course make sure they don't say it was initiated by you), sometimes coming from someone besides the parents might help him see that others think this way about his girlfriend. If he really values what you and others close to him says he will access his current relationship.
Sorry for it being so long... I am just thankful that my brothers chose wonderful girls otherwise they would have gotten an earful from me. Hope that the outcome of your son's relationship is happy, maybe she will change (long shot but possible) or maybe he will see what others see and end it. I guess in the end it will be your son's choice so countuine to be supportive but there is no need to be super nice if it is a person you do not like, you also have the right to not like someone but still have to be respectiful of that person.