Assuming there is something 'toxic' in the jab (thus de-toxing) he should just take lots of water and binding agents (e.g. Activated Charcoal) I'd imagine. Do be careful though as Charcoal will also absorb any medication he may be taking for other things (and also will absorb nutrients so you can't take it for weeks on end)
B-vitamins (of which niacin are a part) are water soluble so you can't really overdose on them. You also can't overdose on Melatonin.
No clue if either of these are in any way beneficial to jab related illness... but if you are worried about overdosing it's not really feasible
Niacin is typically used for energy whereas melatonin is typically used for sleep so it feels like this is self-contradicting.
Assuming there is something 'toxic' in the jab (thus de-toxing) he should just take lots of water and binding agents (e.g. Activated Charcoal) I'd imagine. Do be careful though as Charcoal will also absorb any medication he may be taking for other things (and also will absorb nutrients so you can't take it for weeks on end)