Showmanship? With his current attitude, unlikely without A LOT of work. Only sometimes walks right at your shoulder (he likes to be ahead or behind)
You MUST correct this. And with his appy-tude, you must stay ontop of it. No cheating. He doesn't 'sometimes' get to walk ahead, or walk behind. You have to teach him that he absolutely has to stay at your shoulder. If he starts walking ahead, there's a few things you can try. Push him into a circle away from you, and try again... keep making him circle (like you're doing a pivot almost) until he gets tired of these games... or, if that doesn't work, give him a good yank on the halter without a stud chain, and back him up a bit. If he's still resistant, use a chain, over the nose.
Takes a few seconds to stop...
Again, just poor training. Teach him 'WOAH' means 'WOAH'! Every step he takes forward, he takes two steps back. Eventually, when you stop, he'll expect to be backing up, and he'll stop dead in his tracks. Again, you can try it without a chain, but if he's stubborn you may need to get a little more authority with a chain.
Can I get him more enthusiastic about work?
Yes. I taught my horse showmanship through his stomach. Granted, you have to know when to stop rewarding with cookies - they tend to search for them after awhile. Also, showmanship should be schooled for only a few minutes a day. Practice squaring up after stopping at gates, or leading him to his stall. Practice pivots when you need to move him.
Showmanship is probably the easiest discipline to maintain because it takes the most miniscule amount of time once they 'have it'. Sure, he's older - but you can work with him as much through out the day as you please, just make sure you don't sit down for an hour and do nothing but showmanship. It's like math homework for horses, and they tend to learn to hate it when you do that.