Most countries nowadays observe copyright as innate to a creative work, so even if the composer does not put a notice of copyright on his works, it is there inherently anyway, and he also has moral rights as well. Some countries allow a creator to waive his or her rights, but this is the exception rather than the rule. As such, then, we can’t really accept these as “public domain” since the composer could turn around tomorrow and change his mind on their status: which would be well within his rights to do so.
That said, of the CC licences allowed on IMSLP, the CC-by or CC-by-sa licences are fairly close to public domain. Both insist the composer be attributed for his work; the latter type adds that derivative reworkings should be shared under the same licence type (“share alike”).
Neither of these licences prevent commercial exploitation of the work, however. But maybe the composer is happy to allow that.
If you’re interested in IMSLP hosting the piece, why not drop him a line (a) asking permission (b) exploring whether the “public domain”-like CC licences would be suitable, as surrendering his work to the PD is a legal fiction.
The closest to public domain licencing under the CC system is the CC-0 licence, which doesn’t even have attribution as a condition of use. We’re not hugely keen on that one as an outright gift to serial plagiarists.