Make him tan the word ¨SISSY” just above his penis and maintain it for the whole summer long. Write the word "sissy" nicely in 50spf sun block just 2-3 centimeters above his penis, and make him tan in such a way several days in a row, so that it becomes a semi-permanent sign.
This will be private/secret semi-permanent and frequent humiliating reminder of his status you. This a must have for him!!! It's easy to do, and very intense psychologically. No one should every really see this part of his body except for you, and if they do, then they should definitely see the clearly marked "Sissy" sign.
You will need to start the process when he's in chastity and you need to tell him you will only be releasing him after he's tanned "sissy" just above his penis for at least 3 days in a row. Explain him that he's required to maintain it as long as you say, and give him a timeline at least through a whole summer. Tell him that if he fails to keep it then you will consider making him get a tattoo of it, so he better be good about keeping it as long as there is sun for tanning or tanning booths.
While he's tanning the word "Sissy" just above his bikini line, you should get him in the habit of tanning with a bikini bottom, so that clear feminine bikini tan lines become visible when he takes off his panties as well.
Make him tan this way whenever he's alone or with just you in the sun (not in front of other people). If he can't get sun alone, then make him go to a tanning salon.
You need to make sure the word is consistently written in the same exact spot over and over again for several days, so that it develops the timeline.