Stupid question but is Nexgard off the table for him? My girl did outgrow much of her sensitive gut and I know you're still working on Gandalf's, but she tolerates Nexgard just fine.
I've heard of amber collars being used for fleas but that won't help you with Lyme. I'm guessing your vet is not recommending the Lyme vaccine (which is not exactly highly effective but it is something).
Hope something like the Neem oil works for him.
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