Solution

South is under obligation to bid.
- If he bids a suit without a jump he can do so therefore on zero points. As a consequence his partner, the doubler, will usually pass South's bid.
(North will only bid on holding at least 18 points if he bids a suit of his own or no trumps; if he raises South he can do so on a bit less.)
- If  South does a jump bid in a suit he shows 8-11 points; such a jump is not forcing.
- South's no trump bids show equal strength as the ones after partner's opening of one in a suit. There is a difference though: after partner's double South's 1/2/3NT require a heart stopper, West's opening suit.
- The only forcing bid South can do is 2♥, a cue bid (a bid in the opponent's suit).

South 1 is far too strong to bid 2♦. He would show 0-8 points. He should bid 3♦: 8-11 points, at least four diamonds, no four card suit in spades (major suits have priority, see South 2). 3♦ is not forcing. If North has a minimum for his double he will pass.

South 2 had better bid 1♠ instead of 2♦. In view of his double North will usually have four spades (the other major). So that is the suit we want to play in. South shows 0-8 points. There is a case for a 2♠ bid by the way. After all, assuming there is a genuine spade fit, there are in fact 9 points. 

South 3 bids 2♦ alright. His hand is weak and he has no choice but to simply bid his longest suit. Take a good look at this hand. Too many North players would now carelessly bid again, holding hardly any extra values, hoping South to hold a nice hand after all. Be careful! South might have held three points less; still he would have bid 2♦...