Asdjfhjksdhf absolutely yes oh man!!
Eggsy genuinely does want to return the favor, but it’s also an excuse to get his hands on Harry, to touch him in some way - Eggsy’s halfway into falling in love and he’s touch-starved and always craves Harry’s attention. Between Eggsy’s continued training and the missions they’re both sent on, there aren’t as many opportunities as he wishes there would be - so an excuse, any excuse is a welcome opportunity.
He fantasizes, as he rubs Harry’s clothed shoulders, about unbuckling his shoulder holsters and peeling him out of his French shirts and pressing him into the wall, anchoring Harry with his hands - or maybe Harry’s got him against the wall, using his broad shoulders to pin Eggsy in place - or maybe Eggsy’s got his hands on Harry’s shoulders as Harry fucks into him, clinging as best he can under the rugged force of it.
And he’s snapped out of his fantasies by Harry making a low, dark sound in his throat, something perilously close to a groan as Eggsy keeps rubbing his shoulders, digs his fingers into the meat of them and smooths out the knots - and it’s really not appropriate to pop a stiffy while giving a massage, though it’s apparently somewhat normal to get hard while receiving, and there’s no way Eggsy can surreptitiously glance at Harry’s lap from over his shoulder without being quite obvious about it.
But that’s only the first time - the second time Harry’s breath hitches just as Eggsy’s warm hands make contact with his shoulders, and the third time he does make a bitten-off groan, and the time after that he stills Eggsy’s hands on his shoulders just as he starts to work the corded muscle at the nape of Harry’s neck, and for the first time since his first time, Harry offers to rub Eggsy’s shoulders again instead of the other way around.
Eggsy, unlike Harry, really can’t keep any semblance of control when Harry’s hands are on him, is hard and practically moaning only minutes in - and, well. Harry’s shoulders turn out to be great for holding on to, for snogging and quite a bit more, when Eggsy licks his lips and dips his head and tells Harry don’t stop there.