I have to go to Milan in September for business and need some suggestions on where to stay. I haven't been to Italy in 7 years so any suggestions would be great.

Depends on where your business are. You have lots of hotels around the Stazione Centrale zone, the Fiera zone and the historical city center.
There is a very good metro system that brings you in 5 minutes from the Central Station to the City centre (Duomo), so distance isn't really an all decisive matter.
Where do you have to be, and what is your program in Milano?

We will be in Milan for about 4 days. We are going to a trade fair at Fiera Milano. I don't know how close this is to the city, but I would prefer to stay somewhere central.

My father travels to Milan quite frequently for business, and he likes to stay in the Jolly hotel near the Duomo.

I was some years ago at Fiera also (for ITMA textile show) and stayed at Hotel Domenicano, if I remember correctly. Very good hotel, in a residential zone near Fiera. From hotel you can walk 5 - 7 min. to Fiera. The metro station is veru close to the hotel and in max. 7 - 10 min. you are at Duomo.

I think that is better to stay near Fiera if you should be daily there.

Thanks for the advice, I will take a look at both these hotels on the internet.