You should eventually speak to an expert. When? It depends on how quickly you expect to yield net revenues and need to distribute profits.
If you expect to run in the red for a while and once you show some traction may bring in investors, then I would follow my "rule of lazy paperwork":
1 - An outside investor will want you to restructure into an investment compatible vehicle, not an LLC most likely. So treat you LLC as an operating vehicle while you bootstrap. The less complex things are in the beginning, the less complex it will be when you need to restructure.
2 - For any startup partner you can write a simple contract that says the intellectual property ownership is shared by your persons and will be injected into some new entity in the future. In the meantime, you create another one-page contract that licenses this jointly owned IP to your operating LLC.
3 - You can't easily have both UK and U.S. rules. I suggest your partner accept that he is enjoining a U.S. (really, your state) ruled set of contracts and live with it.
4 - In the future, you can assign your IP into an offhore entity (e.g. Caymans) and then redeploy those licensing contracts your LLC already has. At this point you would also make the U.S. LLC a wholly owned entity of the offshore entity and have all you and your partner's shareholdings in the offshore entity.
The above gives you an operating LLC so you get some protection and can have bank accounts, etc... Until you need to distribute any earnings, you can keep it simple and with little paperwork.
As always, IANAL, this is simply how I might approach the problem.