It is possible for a single member company to change to a multi member company. All they have to do is file an M2 form and make the necessary amendments to their Memorandum & Articles of Association. They also have to file the B5 for whatever shares are allotted to you and a B4 Form if they are making changes to the share capital(if by your subscription they are creating a new class of shares) Its actually simple from a company secretarial point of view, but what I would be more concerned about is what the rights attaching to the shares are.
What you really need to know if he is offering you a different class of shares than those already held by X Limited, and the varying rights attaching.
I would definitely get a Solicitor to review any share agreement before proceeding.