Any ideas how to make this...my sons birthday and thought I can make him this cake as he loves hulk.

no idea how to go about it at all.

It will be whipped cream.

depends on the look you want.  If you want a solid look, do buttercream.  My preference is piping gel.  Maybe this video can show you how piping gel goes on to smooth whipped cream

heres a video for buttercream transfer

Here's a video for a frozen buttercream transfer