I would suggest you play something like d4 e5 the englund gambit? It might take him by surprise
A good way to tackle the London System?
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The problem is 3. ... c5. It seems to set White some problems but after 4. c3 White concentrates on development and gets a slight edge. All this according to Jonsen and Kovacevic's "Win with the London System". The normal follow up is
1. d4 Nf6 2. Nf3 g6 3. Bf4 c5 4. c3 Qb6. The problem is after 5. Nc3 Black misplaces his queen, since the major threat 5. ... Qxb2 is met by 6. Nb5.
Suggest you play a more normal King's Indian with Nf6, g6, Bg7, and d6. Then play for the e5 break.
I have an important game this thursday and would like someone elses opinion on a line against the London System i have been preparing. 1.d4 Nf6 2.Nf3 g6 3.Bf4 c5!? I really dont want to get stuck up in a drawn middlegame and also want to get him out of his comfort zone fairly quickly. Please help!