I really feel you on this, starting solids and teething at the same time is already so much to navigate and it makes sense you are watching everything closely. Poop does change a lot once solids come in, so some difference in colour and texture is usually expected, but since his milk intake has also dropped the last couple of days, I think it would be good to bring him to see the PD just to be safe and get some peace of mind. The PD can take a proper look and let you know if anything needs attention, especially with the teething going on at the same time.