Don't mention the erection at all. As he is about to be smothered in breasts, switch to descriptions of everything he is using to mentally distract himself from having a sexual reaction.

To make it more of a "battle", attacking one of his senses at a time. First the image of her breasts up close which he fights back with other visual imagery that is very un-breastlike. Then as he seems to have that won, his next battle is her smell which is layers of feminine things like soap and powders. He battles this back with memories of foul smells. Last it is the feeling of contact, skin to skin, and he has to knock it away with very un-sexual human contact. (Bonus points if the replacement objects resonate thematically or have character-arc significance.)

After this epic battle of his senses, the reader can get a hint of how well it worked the same way he will know. He "adjusts" how he is sitting.

Don't mention the erection at all. As he is about to be smothered in breasts, switch to descriptions of everything he is using to mentally distract himself from having a sexual reaction.

Perhaps, make it more of a "battle", attacking one of his senses at a time. First the image of her breasts up close which he fights back with other visual imagery that is very un-breastlike. Then as he seems to have that won, his next battle is her smell which is layers of feminine things like soap and powders. He battles this back with memories of foul smells. Last it is the feeling of contact, skin to skin, and he has to knock it away with very un-sexual human contact. (Bonus points if the replacement objects resonate thematically or have character-arc significance.)

After this epic battle of his senses – which of course was just a few seconds but in his mind lasted an eternity, the reader can get a hint of how well it worked the same way he will know. He "adjusts" how he is sitting.