What teets to use to feed my 5 week old son?

My son is 5 weeks old. He was 9 days late and born weighing 8lb15 and measuring 57cm in length. He was weighed again nearly 3 weeks ago and was 9lb13 so is obviously growing fine. However, sometimes now he gets angry when feeding and I think its to do with how much milk he's getting. He is bottlefed using Tomme Tippie Closer to Nature bottles and is on the slow flow teets. Sometimes he takes only 4 or 5oz but other times its 7oz and this can take up to an hour to do. I bought the vary flow teets today but he was dribbling a lot down his chin. I am unsure wether to keep going for another couple feeds and see if he gets used to them or wait another week or so and try him then? Any suggestions?