It's a relief to know that the links could be from those sites.
The thing is this guy has worked for me for a long time and his work used to be really good. But last few months he's either slacking or something else is going on. I may end up firing him since he's focussed only on link building and all on page seo has fallen on me. Blog posting was almost not happening at all on the website and I've started it again just this week onwards.
I might hire some reliable multi tier link building service here rather than outsource it to these guys since they've not been giving any results lately.
it sounds to me like the guy doesn't care anymore.
Ask him for reports on the number and types of backlinks he's built, because if he did stop caring about your site, then you will probably see links from web 2.0s (blogger, weebly, tumblr, etc), article directories, wiki sites, bookmarking sites, auto-approve blog comments, and so on...
Actually, take a look at the RDs and see what kind of sites are linking to you because if your site was not affected by the last google update then the quality of the RDs could be the issue.
Also, like other people have already mentioned, the anchor texts could also be an issue. If your keywords stop ranking out of the blue it's pretty common that the pages that those keywords were targeting have backlinks pointing to them that have too many exact match anchors, which is one of the common indicators of SEOs not caring about their job anymore.
And for God's sake, start posting content regularly. Unfortunately, blogging has become a tedious chore and you have to post content regularly (few times a week at least) to keep the mobster google happy. Besides, without a lot of content you won't rank for many keywords anyway
Anyway, ask your SEO guy for reports on the work he's done... or at least talk to him in a friendly manner if you've been partners for so long and are close to each other, and see what his opinion is about your site losing rankings. He might actually be onto something and working on it
And if he's not helpful, take a look at the RDs your site has, and also check the backlinks and the anchor texts for spammy links, bad RDs, overoptimized anchors, etc... I can't give better advice cause I don't know your site (but don't bother making it public cause I'm not offering SEO consultations anyway), but with the few pointers I gave you should be able to identify the issues if you know your site inside out. If you can't identify the issue then most likely you've been hit by the May update