I would say to watch him closely if you leave him with the ewes. Because their hormones are different at different stages of pregnancy, tups can get a bit aggressive with pregnant ewes, especially close to lambing. As harmony says, Herdwicks should be lambing outside, but with the tup there too, they will get no peace and he might take to butting him. So watch him and at the first sign of aggression then take him out or he could damage the unborn lambs. And as others have said, there's the thing about not feeding tups the same feed as ewes.
For future years, you could keep back a wether to keep your tup company, then send him off once the following year's lambs are due to be weaned and keep another wether back.