Firstly perhaps you should search in future this argument about PHP vs ??? has come up a lot especially with ASP.Net.
Firstly you cannot compare two different things, its like comparing a apple to an orange, its just not possible however you can give a personal opinion on the matter.
Secondly do not argue with your boss no matter how wrong he/she maybe, however you should probably ask him how he got information on .NET being more secure then say PHP, mainly it comes down to the developer using their tools which really decides whether the application will be secure or not, not the language itself although it will try and put some security precautions in place.
If your developing for Windows a lot of the time, ASP.NET is better suited than PHP in my opinion mainly because .NET is aimed for the Windows platform whilst PHP was designed for *nix systems.