Such a lovely set. Engine seems close to unplayable on regular draws, but he doesn't get regular draws. I fully agree with his plan, just not with his execution.
He starts with Pawn/Marchant Ship/Mint/Develop, against your Develop/Silver/Silver/Silver.
On your turn 5 you donate him a Silver, and now it's time to think.
Rushing the Cities is nice in theory, but they cost $5, and until you get them all they basically do nothing.
So my plan would be to get some cities but rush the pawns. You can get them much, much faster.
Turn 5 - get a pawn, not a silver. Silver is an ugly card in an engine, especially one struggling for draw.
Turn 6 - get a pawn, not a storeroom. Storeroom is a strange buy I think. What does it accomplish?
Turn 7 - get 4 pawns, not a city.
Turn 8 - get a city, not an embassy. Embassy looks nice because it reads 'draw 5' but it's quite disappointing in an engine.
Turn 9 - get 4 pawns again, activate your city and soon get more.
Off course I don't know how a different turn 5 would affect the rest of the draws, but it sure feels like a better (and winning!) plan.
There is no need to fear that you would also be going cities after he activates them. You have been buying money all game, and his engine is much more compact.
If you don't go for cities, he probably shouldn't buy more then 7.