Need help clipping 5 mth old OEM nails.?

We have a 5 month English Mastiff puppy. I took him to petco the first time we got his nails clipped. I had to help the groomer and she didn't seem to know what she was doing. So I bought my own clippers and decided we would do it at home instead. The only problem is, he is freaked out by the clippers. I have to do 1 nail at a time while he naps. Of course then he wakes up and I have to wait a while to do another one. So far I have clipped maybe 4 nails in 3 days. Any suggestions?