My 5 month old is congested in the chest, how do I make him cough so the junk comes up?

You can't make them cough it up, all you can do is help it break up a little, a steamy shower room, a bowl of hot water in their room with vics or something similar in (out of reach!). I also use mild baby chest rub suitable from 3 months and Karvol + tilt the cot using either cot blocks or a pillow under the cot mattress (not on top of) My LO is really badly bunged up so I can sympathise with you, good luck it'll soon p.