First ball they come on switch hit him for six. He'll lose confidence and not know where to bowl after.
But on a serious note, as the senior says, cut shot will get you out more than not, so when a right arm off spin round comes on. Leave it out of your armoury.
Rest depends on where your strong after.
If he's not turning it much(and you have decent umpires) lbw should be out of the equation. So you could move 1 guard across to the offside and look to play more through leg if he keeps bowling straight and your good off your legs.
If hes pitching everything outside off, then a big stride to get your pad outside the line and a sweep could also be utilised. This is a free shot as if it hits you outside the line you shouldn't be given lbw.
I'm right handed, and and we have a left arm round off spinner in our league, he aims to hit top off or the "4th stump" and rarely turns it much, he took a lot of wickets in his first few years, but took about 5 last year as most of the league realised lbw are out of the equation. And just get there pad out there and either sweep or nudge/nurdle to leg.