I guess this is why work relationships are invariably discouraged by management...they're just so damned distracting! I have great sympathy for your situation and I believe it's one that many women are deeply familiar with but I implore you NOT to raise this matter with him, not now and not in a year's time, if you're still together, because it almost certainly have the opposite effect to the one you desire - it will make him MORE distant towards you at work and maybe even less loving in private. In truth, I'm not sure there's an awful lot you can do to shake up your boyfriend's attitude at work - if 'closed and distant' is the mode he's most comfortable with, I'm not convinced he can be changed even through the cleverest and subtlest manipulation! The only person you can work on is YOU.
You need to find a way of making peace with this situation and to stop seeing your boyfriend's lack of office attention as a sign of his dwindling feelings for you. Yes, it would be nice if he were the type to drop little love notes on your keyboard 10 times a day, but that's not his style, end of! It's not personal. It's not a sign that there are other female colleagues he'd rather be with. The intention behind his remoteness, almost certainly, is to maintain an image of flawless professionalism. He probably feels observed by his colleagues and doesn't want them to witness the 'leaking' of his personal life into his work life.
Generally this distinction between 'private' and 'public' is more important to men than it is to women. Women are usually quite happy to give a colleague they trust access to other areas of their lives, while men are far more likely to keep work associates at arm's length.
I'm sure this is somehow related to OUR talent for multi-tasking (an example of this: while we're working out at the gym we're also busy thinking about the petrol situation in our cars, worrying about a lonely friend, puzzling over Sainsbury's closing times and trying to remember when the cat is due for her vaccines) versus THEIR knack for compartmentalising the various aspects of their lives (so when they're thinking of sport, they're thinking of sport; when they're thinking of work, they're thinking of work; when they're thinking of sex, they're thinking of sex, etc).
Your new lover obviously finds it both easy and necessary to keep YOU, his treasured girlfriend, separate and distinct from YOU, his work peer. Try not to make him pay for this. Tough though it may be, force yourself to emulate his behaviour. Deal with him in a kind and respectful, but also efficient manner at work. Stop tracking his every move around the office. Forbid yourself from dissecting his every exchange with other female colleagues. Give him the space to be the man you first observed across the office floor and fancied from afar. So long as he continues to lavish loving attention on you out of hours, you really have no reason to crucify yourself with insecurity and worry. So please don' let your fears ruin what could be a wonderful relationship.