I'm stepmom to an 8 year old boy who lives with me - who's biological mother has BPD, he has BPD. I feel like you have no choice but to take him for an evaulation. Stressors bring on cycles; they DO start early - hard to put a finger on it and call it BPD - but close monitoring by a professional is necessary. 'Trying' to smother your baby - may be a success in the future. This is a horrible reality - but your son may not be aware of his strengths, or in line w/ his emotions at the time. A professional opinion may help at this point.
I look at it like this - if you thought there were a possibility your 5 year old couldn't see well - wouldn't you take him to the eye doctor? Same goes for BPD - if you think he may be genetically predisposed, then take him to start some evaluations. End result may be that your doctor says therapy and close monitoring at home is all that is nec. at this time - but isn't worth peace of mind for your family AND your son?
Good luck - feel free to send me a private message if you want to.