constipation 2 almost 3 months old

what can i give my lo he is lactose sensitive and we just switch his milk but he haven't pooped in going on 8 days my mother in law keeps telling me prune juice but i am waiting until his appointment on wednesday but he is in pain so is there anything i can give him now until his appointment.

it's sunday and monday they are not open

thank you guys i talked to a on call nurse

im a ftm so i new to all this