It's good you found a solution all by yourself, no thanks to your doctor.
I'm wondering why he can't latch on well and yet is doing well with the nipple shield. Small mouth/big nipples mismatch?, high palate, tongue tie...
My dd had trouble latching cause of a high palate and basically once she'd grown out of that (i.e. her mouth got bigger so it wasn't such a problem), then I didn't need to supp with syringes, bottles etc. It took her about 4 mo but hers was a bit extreme. Most babies I've heard of grow out of these things sooner.
I used syringes, shield and bottles, although my dd never really took to the shield. She did get nipple preference though. When I was trying to get my dd off bottles and back to exclusive nursing, I'd try nursing at the times she was most alert, in a good mood, not too hungry etc. Pick a good time and try nursing. If he gets too frustrated, try walking around with him, or something to calm him down and try again. If he totally loses it, just let him have the shield. That kinda worked for my dd but finally to get her totally off the bottles, I just went cold turkey and ditched them. I waited til her weight gain was steady to do that and I rented a weight scale at home and monitored her weight gain. The first time I tried it at 6w it didn't work and the 2nd time I tried cold turkey at 16w it worked fine. Her weight gain slowed down a bit but was within acceptable limits, that was expected.
I know in the middle of these things, you wonder will you ever get rid of the thing, but nearly all babies seem to grow out of it.