awhh rebaby.. im sure its just a wobbly stage that you will come out of.. i had a few at the start.. im actually having some breakfast issues at the min, but thats a whole other post!
i agree that the coordination is probably a factor here... ruby used to get quite frustrated at the start, now she rarely does it.. have you tried him in a regular highchair, she also didnt like to eat in her bumbo, and we found she was much better in her big highchair..
also.. is he teething? when Rubys gums were / are sore, she will throw her food away and get very upset, i think it hurts

rubys never really been a fan of the preloaded spoon, she can do it, but she would rather not lol.. and she still sometimes does the goldfish 'no you feed me' face when shes having a lazy day, i just ignore it, and she soon gives up and pick sit up herself!
rubys only recently started wanting to drink from a sippy cup with a lid, but has always drunk great from her doidy, so hopefully once he gets used to that he will be drinking more water..
i think if hes having at least one pooh a week, they dont consider it constipation, and it was also a big shock for us when ruby stopped going 2 x daily, and went to once every few days... but that did even out, and its probably daily 95% of the time again now..
but if you are worried, some fruit / juice is normally enough to do the trick..
its still early days for Toby, hes only just 6 months now, so hes had a head start on lots of other babies, so hes doing good.. it will come! i wondered at several points if i was doing the right thing, but now i know we did, and hopefully it will be the same for you xx
hope some of that helps.. its a bit disjointed but im sleepy lol x