Is zakat obligatory on a single gold coin of the "George" type given by a mother to her son, and how is its zakat calculated, and does it reach the nisab?

The nisab (threshold for zakat) for gold is approximately eighty-five grams. If the gold coin gifted to your husband does not reach the nisab, then no zakat is due on it.

However, if the gold coin reaches the nisab (85 grams or more) and a full lunar year passes while it is in his possession, then zakat becomes obligatory on it. The method of calculating its zakat is to give one-quarter of one-tenth (2.5%) of its weight in gold, or its equivalent in circulating currency. This applies if the gold is not intended for trade.

But if it is intended for trade, then at the end of the year, its value should be assessed, and zakat on its value should be given if it reaches the nisab.