"When I asked the vet for advice she suggested not to castrate him till at least 18 months as it would arrest his development and we would be stuck with a basket case for life!"
I'd seek a second opinion on this, to be perfectly honest. My old dog, a working gundog Springer Spaniel was castrated at 9 months old - and he was the calmest dog I've ever had in my life (and I have spent 45 years with dogs - literally, there hasn't been a time in my life where I've not had at least one dog trotting along at my heels... actually, true fact: my parents GSD bitch taught me how to walk by letting me grab her collar, haul myself upright and totter along as she slowly paced across the living room floor!). He could so easily have followed his breed's traits of being " a nutter", but the castration at 9 months old (which I had done because my mother owned his female littermate and refused to have her spayed) calmed him down to a point where he was, without doubt, one of the best dogs in my life. Placid, easy to train, protective of my then 2-year-old son and 10-year-old daughter as they grew up with him.
The longer you leave the castrating of a nervous dog, I'm afraid, the more inclined they are to escalate their anxieties once "done". My mother currently owns a 5-year-old intact GSD whom she cannot now castrate, under the vet's advice, because he is already reactive. Please talk to your vet because you do not know the genetic background of your undoubtedly lovely dog, and it would be a disaster in the making if he were to impregnate a bitch on heat (and that's easily done even if they don't "tie" - my Lab/Whippet cross and GSD had 3 pups which were referred to as "the immaculate conceptions" because the first day she showed signs of going into heat, at 10 years old, no previous litters... the GSD was shipped over the road to a family member's house! 9 weeks later, my mother took my childhood dog to the vet for advice about the diet only to be told that she was mere hours away from whelping... so it does happen, just as it can with any species). My Kokoni is below knee height, for example, with stubby little legs and a long back, suggesting Corgi somewhere... but she also has a Collie's head and ears, suggesting a larger dog lurking. When my daughter bought her as an 8/9 week old pup, she was told that she was 1/8th Westie and the rest Jack Russell terrier. Nope. She also has a Pomeranion's tail so... who knows what her genetics have lurking. I don't know for certain, just as you don't know about yours. But the earlier the castration, in my experience, the calmer the dog. The longer you leave it, the more neurotic they become because they suddenly feel as though they cannot adequately protect their humans... and this makes them reactive.
Your dog is already reactive - and young enough to curb it/help him to lead a stress-free life... and, actually, he may be reacting to your husband due to swells of testosterone going through their systems. Or, there are certain times when your young dog has bursts of hormones - and views your husband as competition for your attention/affections. That might be worth your considering.
As the PP suggested, keep a bark diary; determine if smell or certain items of clothing are triggering your dog, but also please understand that somewhere along the line, even if only weeks old, your dog is highly likely to have either been hurt by a man - or witnessed his mother being hurt by one. They really do not forget (I used to foster kittens and the one who came to us at 3 weeks old, having been separated from his mother by her male owner - another unwanted/surprise litter - loathed men to the point where his nickname was Psycho... because he would chase after them with teeth and claws. I ended up with him in my life for 22 years, and he adored my son... but he was the only human male he'd tolerate!).
I genuinely wish you and your dog the best of luck. To be prepared to keep working with a street-dog/foreign rescue is the only way they will have the chance of a safe, happy life where they experience love. But you will have to be prepared to spend the next 10 years or so constantly, consistently working with them, trying to get them safely through their triggers and fears, and understanding that your dog may never take to your husband 100%. 