Rabbits will breed in 30sec.or less so keeping an eye on them would not work so well(it would happen to fast.
I would suggest getting one of them spayed/neutered...but only one of them cause it would be a wast of time and money for both. Spay/neuter... Whichever one is cheaper
Only spay/neuter...
You should worm him...the worms could be eating the food inside of his belly therefor he would not have any food and would be loosing weight... Hope this helps! Best wishes~Cocoa bunny