If you are asking how to put in a conversation for 吊針,吊鹽水 in a layman term, you could say:
The doctor is going to put him on drip.
or you could say:
He is on drip for glucose or sodium because he is unable to eat or drink now.
Normally, the doctor will know what are you trying to say if you use the terms [on drip] as this is a very common layman terms to use in the hospital.
As to injection是否只是屬於打針/ 注射? Answer is yes.
There is another informal word for injection is [jab]. For example:
The doctor is going to give him a jab on his buttock (informal way)