Some dentists don’t place silver fillings at all, maybe your dentist is one of them and that’s why he’s telling you you can’t get silver fillings.
The only time a silver filling wouldn’t last as long is if the cavity preparation isn’t deep enough or retentive enough, but this usually is not the case. If I were you I would probably consult with a second dentist. (I would opt for silver fillings for myself over white fillings).