Oh you poor thing, and your poor ds.
My ds (5yo) has sporadic periods of anxiety, though not to the same level. I kind of see where your GP is coming from in that sometimes playing something down can be actually reassuring to a child, whereas sometimes by giving lots and lots of reassurance it can actually fuel an obsession. Eg my ds went through a long phase of huge anxiety about toilets and toilet training. We tried many tactics over the course of a year, and eventually realised that the most successful approach was a matter-of-fact no-nonsense one. The more we tried to give him reassurance, the more strategies he came up with to avoid using the toilet (hated the pipes, scared of the flush, wrong seat colour, wrong seat shape, scared it would overflow, scared it would block, etc etc). The more we reassured him on each point, the more he would get us engaged with the obsession.
Sorry, long-winded, but I spose I'm trying to say that, although of course it's totally understandable and natural for you to reassure him every time, perhaps this is providing more fuel for him. Have you tried saying once, quite brusquely (but not unkindly) "ds you're fine. Now, have you seen this xyz in this book here" or something similar - ie distract him immediately with somehting you know he really does love.
Other thing you could try re bedtime - start a gradual countdown with him of how long you're going to spend in his room, and reward him each time he accepts you doing this. Ie
day 1 - stay till he falls asleep
day 2 - stay for 1 hour
day 3 - 50 mins
day 4 - 30 mins
etc until he accepts a simple bedtime, story, kiss, goodnight, and you leave the room. At each step, tell him clearly beforehand how long you're going to stay, what you're going to do, and that if he accepts this, he'll get tokens or something towards his favourite game/computer game/whatever. No punishment if he doesn't manage it, but just doesn't build up the tokens. Stay very calm and kind on it all, but don't negotiate. If he screams and cries and kicks when you leave, just take him back to bed calmly and say "ok we'll try again tomorrow, and maybe you'll get a token then".
Part of his anxiety might be about not knowing what is coming, not knowing what to expect and therefore fear about how to deal with it.
The above might be a day-to-day strategy which might or might not work, but that doesn't mean you shouldn't go back and try and get more support for him - I would definitely do so, as the cause of his anxiety is the key thing (whether a behavioural/neurological condition, or a MH one). And even if you successfully deal with the current obsession, if he has such obsessive tendancies it'll just get transferred to soemthing else unless you and he are given proper support.
The other thing to bear in mind is that he might have such obsessive nature for his whole life, but the important thing is that an obsession isn't harmful to him, and doesn't interfere with normal functioning and a 'normal' life.
Sorry this is very long! HTH though.