If one puts on teffilin and forgot to say the brocho can he say it now?
What should he do if he already took them off?
Shulchan Aruch Orach Haim 30:3
היה רוצה לצאת לדרך בהשכמה מניחם וכשיגיע זמנם ימשמש בהם ויברך דליכא למיחש שמא יישן בהם כיון שהשכים ויצא לדרך:
"If one wanted to go out on his way early [in the morning, before the actual time that is permitted to put tefilin on], he puts them on, and when their time comes, he should feel them and make the bracha on them. There is no need to be concerned that he may sleep in them since he is getting up to go on his way."
I see no reason why having not made the bracha from forgetting would be different than because it wasn't the (dirabanan) proper time, assuming he was still wearing the tefillin.
Rambam rules (Blessings 11:5) if one forgot to recite a brocho upon doing a mitzvah then if its performance is not yet completed a blessing is then recited. For an example he offers a case when one forgot to recite a brocho upon donning tefilin and states that the person recites the blessing even after already donned.
Pri Megadim (EA 25:12), while not explicitly referencing Rambam, rules that he may recite the brocho after he dons them [as long as he is still wearing the tefilin] since the mitzvah extends for a period of time.
Rabbi Akiva Eger (Resp. Tinyana §13) seems to assume this is a unanimous ruling stating that one can recite the brocho afterward since “he is still occupied with the mitzvah it’s as if it [the recital] is being done beforehand.”
Mishnah Berurah (25:26) quotes the PM, seemingly ruling in accordance, and phrases is it saying “it is a mitzvah which has an extended period of time, a whole day”. So rules the Kaf HaHaim (25:49) as well.