Losec works by stopping the acid production, it is generally the better type of med for reflux. Infant gaviscon works by trying to thicken what is in there so it won't come up as easy, which is different to adult gaviscon which neutralises acid.

I was told 4ml of mylanta up to 4 times a day, if this doesn't help, then to go on the losec. With the vomitting though, like you said it is cause of the weak muscle, so no med will actually stop the vomitting, but will work to decrease the amount of acid in it so that it doesn't burn. It sounds like you need to go back and get a new med prescribed.