Yes, you should be concerned. Rabbits have two types of droppings: regular, firm, round/oval droppings and "night feces" which are softer "grape" like bunches that actually contain benefical things and most rabbits will ingest them so you rarely see them. Now, soft feces is cause for concern. It could be dietary – did you change food recently or get a new bag or what you had been feeding and did not do a gradual switch? Could you be feeding too many greens and such and not enough pellets and hay? Too much alfalfa or rich hay. A good adult rabbit diet should consist of a daily quality pellet ration, a handful of hay and a handlful (less for smaller rabbits) of dark greens or other suitable vegetables and fruits or herbs like sorrell, parsley, lovage,dandelion greens (from the grocer and not the yards as these may be treated). Food that is lower quality or older or even beginning to turn can cause looser stools. Too many greens in a rabbit can cause soft stool. An unbalanced diet can cause this as well. Also, parasites, disease, partial intestinal blockage, etc. SInce this has been going on for a couple days, I suggest strongly you take the rabbit and a sample of the soft stool to the vet for an exam and to check for parasites. If there is nothing medically causing the problem, then it may be dietary. But two days of this warrants a vet call. Also, try adding a bit of live culture yoghurt (mash some with a banana – about a teaspoon or so) each day as well as cutting back on greens for a day and giving just hay and pellets. But seriously, at this point, rule out medical. Once rabbits start to go down, they can go down fast. Often once symptoms of real illness are showing, it may be too late unless you have a good rabbit vet.

That easily could gave done it. Very small amounts of veggies and I would only do a new veggies each week. As he gets used to them, he can have larger amounts. If you notice the soft stool, back off on the veggies for a bit. Good luck!