Basically the only shape/size you want to avoid is choking size, which is the size of a grape. When an LO chews/sucks on a spear, they are learning to manage it on their own. Solids isn't just about getting food into your baby, this whole time they are learning about biting, chewing, and swallowing, so what you are trying to do is give them ample opportunity to practice these skills. Eating it comes after all of these are mastered. If you give a baby a spear of food, he learns how to pick it up, put it in his mouth, chew on it, bite pieces off, use his tongue to move them around in his mouth and to the back where he eventually swallows it. And HE has to learn these things, you just have to provide the food and the opportunity.
If you are worried about choking, please know that it is very rare that an LO actually chokes. Gagging on the other hand is completely normal and is actually supposed to happen. It is how an LO learns to manage the food in his mouth. Also, knowing what to do in the event of choking helps put some of the fear aside.
Most moms will offer spears or small pieces based on their comfort level, but also you have to judge what foods are ok to give this way. For example, a raw carrot or a slice of apple is probably not the best to give in spear form if you are worried, as choking sized pieces can be bitten off. But something like ripe pear, or cooked carrot, or even toast works well because they all turn mushy in the mouth.
How much he eats is not particularly important, what is important is that he is getting exposed to a wide variety of foods. Make mealtimes fun and relaxed, and try not to stress too much (SO much easier said than done!).
It sounds to me like he would do better with Baby Led Weaning. I actually really recommend the BLW book by Gill Rapley for more in depth info. It really helps to understand what weaning is all about and puts your mind at ease with concerns regarding finger foods.
