that is disloyal. in an honorable exchange, he gives you his knowledge, you give him what you know he wants for that knowledge: allegiance. If you do not wish to give him allegiance, then you should not accept what he is offering for that allegiance: knowledge.
The disloyal act here is going behind your teachers back and doing a thing he forbids. Learning from other teachers/ styles should be encouraged and does not mean a break of loyalty.
My advice would be to discuss why you want to train elsewhere from him and listen to his reasons why he thinks you shouldn't. If the two of you can't agree and you still want to train elsewhere then walk away. Shodan isn't so important that it can't wait another few years at another dojo.
I agree. If he merely prefers his students not to go and learn from others before the have a strong solid basis (some would say Shodan or 1st Kyu), it is very sound. If he expects his students to learn only from him, I think you should probably start thinking on quitting at some point (unless he is an exceptional teacher).
My teacher actually encourages his students to learn more M.A. (He is teaching Judo & Karate besides Korindo Aikido and progressed in all 3 M.A. to senior ranks (around 6th Dan)).
I went to train in Aikikai for a short while in another dojo and he just kept asking me for interesting new ideas. I have also trained in TKD for a period (2-3 years) and one of my reasons was my wish to see the perceptions of other teachers. Other students went to train in Kendo and other M.A.
One point to consider is that when you wish to study another style of Aikido, it does imply some lack of confidence in his teaching of THE SAME M.A. and from my experience, you mat find it more difficult to distinguish the teachings and custom in 2 similar Dojo's...