Welcome to the forum Nikki!
I'll tell you more about the pure
Devotional Sex views, but what is important for you and your husband is to discover what works best for you both. If a little bit different from Devotional Sex works best for you both, then you are doing the right thing!
I'm sure that you are not the only couple where you enjoy the Princess role whilst he would really prefer you to be his BDSM Mistress. Devotional Sex then becomes a happy compromise where he gets some of his 'needs' fulfilled and you get some fun and benefits from his being your Knight. Of course there is no need for you to use any of the Devotional Sex words or commands if you prefer your own words.
You can compare training his erection to your both training a dog. He is the main trainer of the dog, but what you do either greatly helps the training or can harm the training. So when you and he both want the dog to be well behaved, and the dog fails, you are both disappointed. Punishing him for ejaculating without permission is like punishing him for the dog misbehaving - not very useful.
Note that a Knight should let his Princess know if he is approaching the point-of-no-return. He should let her know BEFORE it is too late. The Princess then either stops or eases back on the stimulation, or says "Good, I want you to cum now". (She NEVER tells him not to cum and then stimulates him to try to make him disobey.)
So when he ejaculates without permission in Devotional Sex it should be just his disappointment and feeling of failing which is all that is needed to inspire him to try to better train his erection.
Another problem with punishing him is that for a submissive man the punishment may really be what he wants. He is then likely to either deliberately or perhaps subconsciously ejaculate when he should not so that he can 'enjoy' the punishment.
If you really want to train his erection and have him strive to be a better Knight, then it is much more effective to not punish/reward him after an unwanted ejaculation.
One part of being a Princess is to ensure that your Knight stays wanting to be your Knight. One way of doing this is to give him the occasional BDSM style treat. Explore with your husband what treats he would like, then you can decide which of these you might sometimes do with him (and as Princess you can ignore the things that you really don't like).
The Tantric component of Devotional Sex says that the ejaculation should always be enjoyed. But perhaps your husband also has a strong BDSM interest, and he would really enjoy it if sometimes you made him have a ruined orgasm. If so, then you can sometimes tell him that you are going to make him cum, but it will be a ruined orgasm. But make clear that this is a reward
for him being a good Knight - that you are allowing him to prove his devotion to you. Making him eat his own cum
is a popular kink, and there is a section on this in the website.
Once again I recommend that if you make him do this it is seen as a reward, and so it will only happen after a controlled ejaculation.
Remember that the mind of the male submissive is very complex. Being made to do something he does not like can generate very strong submissive feeling that he craves. Don't get tricked into thinking that he is being punished when you make him feel very submissive.
I hope the above is helpful. Please feel free to ask any further questions, and it would be great if you post back one day and let us all know how it goes.