My DS is almost 9 months and I will be returnin...

My DS is almost 9 months and I will be returning to work in 3 weeks time. I am going to take advantage of my hubby being home over the long weekend to introduce formula feeds to his day time routine. Anyway, I dont which formula to get. Does anyone have recommendations. I have heard some cause constitpation and others tummy upsets. Any advice would be welcome. Post is urgent I need to buy the fomula today. cheers