The honest answer is maybe, and that maybe depends entirely on whoever is screening mail the day it arrives.
Mailroom staff apply a subjective standard when evaluating incoming photos. There is no universal rule that precisely defines where suggestive ends and prohibited begins, which means the same photo can sail through at one facility or on one day and get confiscated at another. A sideways silhouette showing only your backside with nothing explicitly exposed sits in that gray zone, and gray zone outcomes are unpredictable.
What is predictable is that lingerie and bikini shots get through consistently. They are accepted at virtually every facility, they accomplish exactly what you are going for, and they do not risk the whole envelope getting rejected or flagged. A well-chosen lingerie photo leaves plenty to the imagination and lands in his hands reliably, which is the actual goal.
If you want to try the silhouette shot, there is no harm in attempting it. The worst outcome is that it gets pulled in the mailroom and does not reach him. But if the priority is making sure something arrives and has the intended effect, the safer and equally effective route is the lingerie or bikini option every time.
Send it through InmateAid's photo service for the best chance of clean delivery. The photos are printed professionally and sent through the standard mail channel, and keeping it tasteful maximizes the likelihood that everything in the envelope reaches him intact.