Wow! 1/4 cup of meat for a 6 month old???? I'd really question if the pediatrician's advice is up to date, considering that the American Academy of Pediatrics reccommends holding off on solid foods at all until 6 months of age--formula or breastmilk should be the primary source of nutrition until age 12 months. Any solid foods you offer just replace the milk with a less nutritionally sound food--basically empty calories, despite what the folks at Gerber will tell you.
Babies don't have the enzymes needed to digest meat until they are about 7-9 months old. And quite frankly, they don't digest many solid foods well in the first year--thus why you feed a baby peas, and have a green diaper, then feed a baby carrots and have an orange diaper...
None of my 4 children would have eaten 1/4 cup of any solid food at 6 months of age, let alone meat. I was fairly quick to introduce table foods to my kids, not doing much at all of the jarred baby foods. My second daughter liked beef barbeque when she was 8 months old, what can I say? ;-) But the solid foods my kids ate at a single meal could be measured in Tablespoons--sometimes fractions of Tablespoons. They certainly were never eating 1/4 cup of pureed meat (yuck!). Even now, my 8 year old is happy with 4 chicken nuggets--which I'd guess to be about 1/4 cup of meat.
On the biter biscuits...try them and see what happens. If he struggles to digest them, just stick to frozen wash clothes or traditional teethers.
Bottom line, follow your son's cues. With two older girls, I'd bet you are lucky to eat your own food lukewarm. Your son will do just fine if you go the route of "you can eat solid foods when you can put them in your own mouth and chew, I really don't have much time for spoon feeding."