Electrolytes are a temp assist to offset body balances if the animal is not eating, drinking, getting nutrients his body needs. Most minis have enough coat to stay warm in very low temps. So, unless he had stood in cold rain and become chilled, weather usually isn't the culprit for a horse his age. Heavy, cold wind is generally not a problem if there is a wind break. A horse standing in snow, WITH a layer of snow on them, is a healthy horse. Yep, their bodies raise the hair at the base and this helps insulate them, thus snow doesn't melt onto them, they shake it off. Minis have heavy winter coats, so I like to feel each one every couple weeks to be sure they still have enough fat over their bones -- they all look plump with that coat. If I see a crease down their back in that nice coat, generally all ok -- if NO crease, you may have a weight issue. This is a very general looksee and in no way the only thing you should check, just an observation of mine that corresponds with amount of body condition you find at topline.
I like to alternate dewormers so, I personally would have used Ivermec in winter, after first heavy frost. It is a good product with a large margin of error on overdose. Quest, on the other hand is a product that must be used quite carefully in minis as the overdose margin is low. A horse with a worm load will most often not have a good coat. I like to see an even, "velvet" look to them.
While I could not guess at what is "wrong" I can suggest that they do need more feed in winter and hay produces more heat in digestion for them. Be sure you are using good quality hay. Then, Spring grasses are generally full of sugars.....thus, question about hot feet as found in foundering. Pastures can look great and still not be sufficient in what an animal's body needs. I like to supplement all the heavily pastured animals with a high protein, vit/min feed. (I use Purina Enrich 32) Just small amount, even every other day. These guys are small and cannot eat volumes, thus my concern that they get sufficient nutrients, without volume. Teeth are often a big concern in a mini as they have smaller mouths but, full sized teeth. Eating can be effected if sore mouth.
These are just what pops into my mind looking at what I read as to his upkeep -- not that anything is wrong with it, just where I would look if he were mine. We are several hundred miles apart and I am not "hands on" with him. You appear to be doing right.
The less than perfect leg movement -- could be just caught "off guard". But, staggering in a horse is cause for concern. So, some common loss of control causes would be ingestion of something toxic, lack of needed nutrients, or infected through bacteria/virus. EEE, West Nile, EPM, listeria, botulisum... come to mind first & can all produce poor gait & or staggering, loss of limb control. Hind legs generally go first.
Most of my herd are elderly now.....I have ten between 28 & 33. Others are late teens to early 20's. I use a lot of extruded Senior feeds (Blue Seal). Can we say "retirement home"?
This has become a book......hope your guy is ok. I just listed things that I would be looking at if such an issue presented to me. Let us know. We all learn.