Declan McDonough tried to make all the running on the UK raider, but looked set for second place when headed by the 5-4 favourite Soul Search.
However, Sir Boris (14-1) knuckled down and got his head back in front in the closing stages to regain the lead and prevail by a length and a half to land the Listed spoils.
The pair pulled four and a half lengths clear of the third horse, Zarzyni.
It was a second notable triumph in three days for Dascombe after lifting the Ayr Gold Cup with Angel Alexander on Saturday.
“I loved him going down, he moved beautifully on the ground,” said McDonogh.
“Once I got him on to the rail and had that to help, he battled well. It’s very chewed up on the inside.
“He’ll probably get seven furlongs. He had made the running twice in England so that was never an issue. Everyone in the race was wanting to take a sit, so I was aware that I might be stealing a few lengths. It worked out lovely.
“I didn’t know much about him, I’d just read his form and watched his races.”
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