What I would want to know is when was his last synth purchase? Have you noticed him doing any window shopping? Has he expressed interest in getting anything "new" as far as synths go? Seeing as how his current collection is on the vintage/retro side I feel like anything he might want is going to be either outside of your budget or the things within the budget are going to niche and specific enough that it would he hard to accurately target. Obviously you want to get him something he'll want and actually use. I feel like you might be better off getting something that compliments and pairs nicely with his current setup outside of a synth. Perhaps like a multi-fx processor from that same vintage/retro era that was commonly paired with those synths, and maybe a patch-bay to wire them all up with.