@mcrelg,
Have I got this right?
The child's mother leaves the child at his/her grandparents house for weeks at a time?
Does the father live there too?
Are the grandparents competent to care for the child?
If those things are true I can tell you first hand that CPS won't do anything.
Someone left a completely unrelated 2 year old at my house 8 years ago. When I thought that maybe, just maybe, someone would be interested to know where this baby was they said "the mother left him in a safe place, good for her" and "if you want us to come get him we'll pick him up and place him in foster care".
Four years after he was left there I adopted him -- that was 4 years ago.
As to the bites are they bug bites? Human bites? That makes a HUGE difference.
Is the rash diaper rash? Is the kid seeing a doctor for anything? Do they simply just not treat it?
I think your best bet is to call the police non-emergency number and ask them to do a welfare check. If the house is dirty and unhealthy they will place the kid in foster care or family care until things improve.
If your boyfriend is the child's father why don't you two take the child in? As the child's father he has as much right to custody as the mother does. Talk to a lawyer.