Just set the client allow download cvars.
Client will then reconnect (either to retry or to ask why he/she was kicked) and download the mod.

IzNoGoD writes...

That's fine if you have menus and stuff that the client needs - they simply can't play as they can't spawn. But for smaller mods, which only have a few items, just setting the cl_allowdownload dvar wont stop them playing. I've seen too many players running around a mod server without the mod items and they are left scratching their heads because they can't work out what's going on. It therefore illustrates that there is sometimes a need to tell the player that they need the mod download.