Is it permissible for a mother to propose marriage on behalf of her son without his knowledge, while he currently has no desire to marry due to his young age and his wish to complete his studies? And is the marriage valid if one of the parties is coerced into it?

If a mother orders her son to marry, and he is unable to do so, then she is not to be obeyed in that matter. Obedience is only in what is good and permissible, not in what causes harm. If he is capable, she does not have the right to force him to marry someone he does not desire. If the compulsion amounts to duress, then the marriage is invalid. However, if the person agrees under duress without being coerced, the marriage is not invalidated thereby. The son should either search for someone who accepts his situation, as a justification to his mother, or convince her of his lack of desire or ability to marry.