There is a 25% chance of having a Thal Major child if both parents are thal minor. A child adopts one gene from each parents. Each thal minor parent carries one healthy and one defective gene, if a child adopts both defective genes from the parents he/she will be born thal major, a child can adopt both healthy genes from the parents and become completely disease free (chance of 25% as well). A child can adopt one healthy and one defective gene and become a thal minor like the parents (50% chance).
In simple words, if both parents are thal minor the thal chances in a child are:
Thal Major 25%
Non-Thal 25%
Thal Minor 50%
In your case, since your wife is non-thal the worst you can get is a thal minor child (25% chance) and chances of a complete healthy baby is 75%.
Please note there are very rare occasions where a chils is Thal Major even one parent is Non-Thal, in such case there may be some other genes come into play.