Mangos!!!!! YUMMY!!!!
Barley cereal, millet cereal (may have to make your own and process yourself, can find the grain at Whole Foods and other health food stores in bulk), green beans. Super Baby Food (Ruth Yaron) suggests yogurt as a first food for babies 6 months and older, although I'm waiting till closer to 7 (any day now, in other words, for us!); she hasn't had any reactions to dairy in my diet, so I'm not too worried about that.
BTW, if he's not keen on avocado, don't give up on it! It's easy to mix with cereal and fruits like pears and apples, with veggies like squash, and with cereals. It's got lots of AWESOME essential fatty acids and is good for brain development.
You can also gently season some foods and food combinations and see how he does with herbs and spices like cinnamon, cumin (both good on squash, BTW), a little garlic, maybe basil or oregano.
BTW, if he's already had oat cereal, PLAIN Cheerios don't have much in them besides oats, and you can get organic ones at health food stores too. Soak in formula or EBM if they make you nervous or if he's not keen on crunchy stuff and let him use as finger food practice!