The 20-year-old, who signed on loan for the Pilgrims last season as cover but never appeared for United, is expected to make his debut against table-topping AFC Fylde on Saturday.
Dixon has already enjoyed a loan spell at Sky Bet League 2 side Hartlepool United this season.
United boss Dennis Greene has moved to secure Dixon’s services with Sam Vince beginning his three-match suspension on Saturday — and losing prospective replacement Nick Draper at late notice.
Greene told bufc.co.uk: “We had intended to utilise Nick on Saturday — but then I got a text message saying he had decided to go elsewhere.
“I am not happy about it because he is registered as a Boston United player and nobody has been in touch with seven days’ notice of approach.
“Thankfully, though, our good relations with Sunderland delivered an excellent up-and-coming goalkeeper for Saturday’s match.”