I'm not excusing anything that has gone on and I don't think the school has handled this very supportively or sympathetically or indeed shown good communication between its staff members.
However, in reception we sometimes get children who have never been in any kind of social setting before and who have very poor skills at dealing with others, particularly when they are not getting their own way. If children have been in nursery or day care before, we do get reports, some of which can be massively unreaslistic in many respects - undiagnosed special needs, behavioural difficulties that have not been addressed etc. So there is a chance that this child arrived in school with no history and the school were unprepared. Let's face it, some children have parents with few parenting skills and the children have not been taught how to make friends or play with others.
My guess would be that your son is not the only child being hurt, as it is very unusual at that age for children to pick a "target" - it is more likely to be general lashing out and lack of control. What happens more usually is that a child who is seen as "naughty" by the others gets the blame for everything, even when they weren't involved. The behaviour is not acceptable, but it is not massively unusual.
What I would expect by now is for the teacher and the school to have cottoned on to this child's problems and to have a plan in place to support him so that things start to clam down. The usual thing a behaviour support teacher would suggest in the first instance is a sticker chart to reward the correct behaviour, probably with "thinking time" when class rules are broken. I'd probably throw some extra lunchtime supervision in there, as the adult:child ratios are very different at lunchtime. As a reception teacher, I'd try to handle things myself during the first term and then get extra help in after that if the usual strategies weren't working.
Of course as others have said, it's not your business to have any input to what happens to the other child. What may be positive is to help your own child build relationships with some of the children he does get on with through regular after school play dates etc. That would shift your child's focus to the positive relationships he has rather than the negative one.