Hi & welcome,
If your horse has been lame on & off for the last month, I'd want to get that properly checked out. If you decide to get xrays done, make sure your vet marks the hooves - dorsal wall, hairline & point of frog at least.
Yes, ground suddenly going hard can certainly exacerbate problems, but it isn't likely to be the root cause. Interested to know, what exactly did your farrier do to 'take the pressure off'? Why did she advise washing with Epsom Salts? Do you mean soaking? Did she suspect an abscess...?
Yes, laminitis is indeed a risk, especially if he's an 'easy keeper'. Yes, your farrier *should* have picked up on it('should' is a great word...
) but they often don't, so not a given if she didn't comment. & to address another comment, while laminitis doesn't affect only one hoof(generally or from metabolic causes) there may be mechanical issues that have caused this one to be more susceptible to further damage after metabolic weakness.
If you would like to post some hoof pics, for other opinions, see the link in my signature for what's required. There are a few knowledgeable & experienced people here that can 'lend an eye'.