What can I do to break up phlegm in a 6 mos olds chest? He has a low grade fever, congestion, low appetite, vomiting, 5 days ago symptoms started.

Fever/vomit 6mo old. You should report this to your pediatrician and be seen as soon as you can for diagnosis and treatment.
Check with your doc. This is very serious for a six months old child. Professional examination asap. Internet not appropriate.
First I would. Suggest that if you have not already done so, have the child see his physician. In addition, cool mist humidifiers are helpful in keeping secretions thin. Monitor the baby for adequate hydration (tears, moist mouth, normal # wet diapers). Do not give the baby water. Infants do not need plain water. There is plenty of water in formula and breast milk.
Pediatrician time. Hopefully, a pediatrician here will reply - but I have a feeling they may also suggest taking your baby to the doctor. Don't use otc drugs on such a young child without a doctor's advice. Do try to have the baby drink water as much as possible to help thin the congestion. Best!