Where in the UK are you located do you have tesco near you? Here's what you need, they sell in pharmacies as wellI will go pick some simethicone or fennel seeds up
Infacol baby pain relief gas drops (simeticone)
Or Gripe water (this is usually dill water for relieving gas)
You will find fennel and dill seeds there as well, Tesco brand is much cheaper but good.
Yes you can use teas with antibiotics, but honestly I don't know what antibiotics you got and why, he is clearly having problems with his digestion and not respiratory, you need to make him poo asap so he can start eating