What should I do?

Ok guys really need some help. Just bought a house and about a week later the wife decided to do 4 loads of laundry and I noticed a wet spot above septic. I immediately called the local septic company and they came out within the hour and pumped everything witch was mostly just water. The septic guy said he thinks the drain field is no good and needs to be replaced. Some background information on the house and septic, the house and septic have been there for about 30 years but was only occupied for about 6 months and then left vacant. The septic is a single 500 gallon tank. Soil is clay. You guys think he is correct?
Do I have any other options?

It is very likely that since the house sat for 6 months that the lines could have dried up and clogged going out to the leach field. It also could be something as simple as the baffle tee going out to field is broken. Did you happen to look in the tank when he was draining it? Also if you are doing more than what the field can handle, it might be a good idea to daylight or drywell your washing machine. Check into the local codes around your area.
Also get a 2nd opinion as septic systems these days are pricey depending on where you live and what type of system that needs to be installed.

Can you see into the septic tank to where the line to the leach field is connected? If so there should be no water above the bottom of that line. In other words, water in the tank should drain out of the tank and not build up or fill the outflow line. If there is standing water in the tank drain line there is something plugging the leach field. Next time the tank is pumped watch the outflow line - if water flows back into the tank from the field the field is plugged up.

Is it a metal tank? The baffle that keeps solids from getting into the drain line may have rusted away. Happened to me....

Are there trees or bushes growing in the leach field -- might be roots blocking.

Good luck.

30 year old system, lateral lines might be clogged with solids. They would need to be replaced in that case. Or could be plant roots if there are large bushes or trees near the lateral lines.

With clay soil the system will drain slowly under normal conditions (this is counteracted by having a larger drainfield to spread the water out over a larger area). If the soil is already wet from rain or snow melt then the system may not drain at all for a bit.

4 loads of laundry is probably too much water for a 500 gallon system in clay soil, on top of all the other water you used that day and the previous couple days.

Try not doing so much laundry at one time and spread it out over a week or so if possible. Don't do laundry or use a lot of water after rainfall or snowmelt, especially if there is ponded water out where the system is (flooding in the yard). Ponded water means the soil is saturated and any additional water added to it will not infiltrate or drain.

Do you have a high-efficiency washer or an older model? Older models will use more water. How large is your house in square footage and number of bedrooms? 500 gallon tanks were common decades ago, but most jurisdictions these days require a minimum 750 or 1000 gallon tank for new systems. Your system may be too small for your house. Some people have no problems with old systems, others do, it mostly depends on system condition and water usage. Was the septic system inspected before you bought the house?