That is not a CR 2 opponent, IMHO, regardless of conjure animals. If you only look at it's AC, HP and melee attack... it's tougher than a CR 2 ogre. +1 AC, +1 average damage, and +32 (!!) HP.
Plus it has disease, immunities, zombie toughness, pack tactics, and a 5th level caster spell-list. So, let's compare it to some other things...
CR 2 berserker? More HP, more average damage, and still all those special abilities. CR 2 priest? Same spell-casting and 1 point worse AC, but better in every other regard - including more than triple the HP.
I'd guesstimate it at CR 4. More dangerous than a CR 3 knight (more HP, spellcasting, other specials - but worse AC), and less dangerous than a CR 5 gladiator (which is superior in nearly every way except the spells). It's also considerably less dangerous than the CR 6 mage (which is a glass cannon; easily killed in a single round, but bad things can happen if it gets a couple of area-effect spells off).
Usually, CR calculations are best made by looking at a handful of creatures on either side and figuring out whether your target creature is likely to be a less or more deadly foe. CR 4 feels about right to me for this one, given that even 1 round of pack tactics conjure animals could be very dangerous (even lethal) to at least one party member. Also, this guy is packing a potentially lethal disease attack and does surprisingly chunky melee damage as well - and will be around for multiple rounds to use it with his very large HP total and zombie resilience.