Make him wear his girly pajamas at night, no exceptions. If he doesn't already have a couple of pairs of very girly pajamas, make sure you take him to get some, or surprise him with some. Enforce that he wear the girly pajamas to bed every night, until he gets used putting them on by default. You can always tell him when you allow him not to wear them, but it should be de facto that he wears them.
The point is to train him to a point whereby his sissyness is by default without you having to ask. The same way he should be secretly awaiting your decision to release him from chastity, he should be secretly awaiting your permission to allow him not to have to wear female pajamas. Out of fear of your reaction, he should never think about wearing anything else but sissy pajamas, even when you are not there to force it.
You should lay out female pajamas for him one night before headed to bed, and then tell him not even consider stepping into bed without them on. Alternatively, you could make a rule that after a certain time every night (say 11:30) he must be wearing his sissy pajamas, unless there is someone else around. Scold him if does not put them on (negative reinforcement), and tell him how ridiculously cute he looks as a sissy when he does put them on (positive reinforcement). You can also use this attire as an excuse to tell him how much of a sissy he is and how he's not man enough to make you excited
Make him wear the full top and bottom of the girly pajamas. If he doesn't have some, either buy some for him as a surprise, or take him shopping and make him try them on. Get a couple pairs, one sexy, one girly-cute, etc.
This should be a rule enforced EVERY night, with no exceptions. You should only allow him not to wear them on few occasions, like one night out of every 2 weeks or if you have guests that might see him (that you don't think he should be exposed to)
Generally only you will be involved in this, but on the occasion that you have female guests over, why not have him put on his girly pajamas and then say good night to them… it's a cute way to humiliate him a bit. You can also threaten him by making him put them on, but then allowing him to discreetly go to bed without exposing himself to another female staying.
For this to work you must be very strict in enforcing it every night until it becomes habit. Often times before sleeping you are both tired, and may not have the effort… but make him put the pajamas on, otherwise you will allow him to develop a bad habit. Be sure his pajamas are sufficiently girly to make the point. Also make sure they are comfortable for him, so you both get some rest.