Whether it's her first, second baby, ALL women deal w/ postpartum issues in their OWN way. Just let her know that she has support and help when she needs it. Make your son realize that her body is going through changes now that the baby has left her body, and her hormones, mood swings, etc. will ALL be crazy for at LEAST the first 3 months. Explain to him that he needs to be willing to let her rest and relax when she really needs it (assuming that he works during the day, have him take care of the baby at least ONCE overnight). Make sure that he knows that HE might have 2 do laundry, go food shopping, make dinner/breakfast, etc.....ask him AND his g/f if it's okay if U drop by once/twice a week just to help out. If U just show up unannounced, it could make the g/f upset w/ unexpected guests....As long as he loves his g/f AND the baby, everything else will fall into place.
Remember how YOU felt when U had your son...did U feel overwhelmed w/ joy? Overwhelmed w/ the HUGE change in your life? Try 2 remember what worked for YOU, and suggest the same things 2 her. Have your son's g/f talk to a Nurse Practitioner @ her OB/GYN's office, even have her speak to the Dr. if she thinks that she needs to do that. It's perfectly fine, nothing wrong w/ that.
Does the g/f have other friends who have kids? Does she have ANY community support (church, Mom groups, etc.)? If not, find out what's available in your area for those resources. Check online (meetup.com, even check on here (mamapedia)), call the OB/GYN's office for help. There are PLENTY of people who your son's g/f can reach out to if she feels the need for support and encouragement. Is YOUR relationship w/ her good? Is it difficult now that she's a new Mom (I'm guessing she's a first-time Mom)? Do U feel close 2 her? Is she close w/ her OWN Mother/sister/cousin, etc.? Even if she's not saying anything, she needs comfort and support that she's doing a great job taking care of the baby. Give her hugs, praise her for how well she does little things...keeping the baby comfortable, changing diapers, etc. As we ALL know, there are NO instructions when the baby gets here and is home w/ us and he/she is SO tiny, helpless, and SO dependent on our time, energy, etc....as long as your son's g/f feels that she has support and love from U AND your son, she'll get through these next few months really well :)
Hope that helps, take care, God bless your new grandchild! I'm sure he's beautiful!