This is disgraceful. This so-called teacher and vile human being is deliberately bullying this young man in a sly and sadistic way. He is using your stepson's reticence to let it be known about his Mum's death to manipulate your stepson's peers and other teachers into being his flying monkeys, i.e. persistently asking your son about his Mum, disbelieving your stepson's account of the teacher's behaviour. This cunt's behaviour is utterly wicked and entirely calculated.
It sounds too that the rest of the teaching staff are failing to recognise the situation. Whether that is wilful or partly due to ignorance of the actual circumstances I don't know. Please listen to other posters' advice about stopping this cunt's behaviour in its tracks. If the abuse were of a physical or sexual nature he'd be suspended!
What is it about emotional pain that some people don't get? We all experience it: grief, rejection, betrayal, loneliness and so on, and it can become a killer if it is not recognised and dealt with in a healthy way. To use a physical example as a metaphor, the "wound" i.e. grief over his Mum's death never healed, it just looked like it had. Instead, it got larger, more complex and is now "infected". He's dealt with the pain by avoiding 'touching' it, i.e. pretending to ignore the "wound" and hiding it from others in case they touch it too, whether accidentally or deliberately. This vile cunt / so-called teacher has sussed your stepson's vulnerability and is fucking with his head. He's deliberately poking that wound in the full knowledge of the pain it causes in and of itself; but is also being sadistically viscious because he knows the boy's pain is exacerbated by his wanting to keep his 'secret' from being found out. This results in him getting into trouble, not being believed and so on. So layer upon layer of confusion, pain, anger, hopelessness builds up in the boy. This cunt is clever. He doesn't even need to do anything much - just press a few of the boy's buttons, watch the consequences and stand back looking puzzled, and say "Eh? Sorry, I must have forgotten to not mention his Mum. Blimey, a bit of an over-reaction or what? Kids, eh? "
The anguish your stepson must be experiencing on top of the unresolved grief must be overwhelming. Young people, especially boys find it so difficult to make sense of their feelings. I'm not surprised he doesn't want to talk about his mother's death - I expect he believes it's just more pain on top of what is already almost unbearable.
The problem is - and he's too young to have had the experience to understand this - that until this emotional "wound" is opened, cleaned out, steps taken to avoid it becoming reinfected, and allowed to heal naturally and healthily, the pain that's become his 'normal' will grow even larger and further infect / consume other areas of his (emotional and mental) health.
I came across the idea of Emotional Hygiene on YouTube. It makes a lot of sense and you might find it helpful. It's a TED talk by, I think, Guy Finch - also a book I downloaded - where he talks about recognising early symptoms of emotional ill-health, when first aid is appropriate and when to seek help from health professionals.
OP, I sincerely hope you and your husband are able to put a stop to this abuse of your stepson, and get him the help he needs to deal with the festering aftermath of his Mum's death. It will be hard if he refuses to go along with it, but perhaps others can suggest ways where he might feel safe enough to start opening up.