It sounds like you have run out of steam which happens to everyone. In relation to your energy levels, are you getting to bed early and eating well? (Sorry, I know I sound like your mother, but every time I think I have no energy it does turn out that I could do with looking after myself more!)
He is communicating his needs to you, which is good. He needs all your time and attention, it is as simple as that! (I hope you don't mind the humour.)
TV is a double edged sword, even if dc are distracted it can have a negative impact on behaviour.
I must admit it hasn't been as structured as it should be lately. We still have a very rigid morning and bedtime routine but in the middle it's a bit of a free for all
So to try to be re-motivating, I agree with the poster upthread who talks about planning things out in advance, because dealing with a toddler is exhausting at the best of times, if you have a go-to list of ideas and things planned it makes it a lot easier to think about what to do and implement quickly.
Another key thing with your LO is that he is developmentallybehind, and so I think you could be gently doing work with him about this every day, so every day set aside some time to:
(a) Get him to repeat word sounds and words - encourage by offering a sweet every time he repeats the right sound a few times, maybe - building up over time - this will help encourage speech, give him confidence and help develop the muscles in his mouth to make the right movements/shapes/sounds
(b) Spend an hour at least helping him to do physical things to build up gross motor skills, like climb a ladder and go down a slide (over and over), maybe have the slide going down into a pit of balls, balance on things, jump of something onto something soft, make sure his diet and sleep is as good as it can be, he is eating a lot to give him energy.
(c) give him the opportunity to use pencils and paints, giving him a free reign to do as he likes, to help with fine motor skills and and encourage self expression.
A really important thing about this age and also up to primary age is repetition. Repetition, repetition, repetition. If we aren't used to this, as adults, it can take a bit of time to adjust. It is reallyhard work for us adults - though worth it in the end. He will really benefit from and enjoy the 1:1 attention too, if you are calm and gently pushing him to do it all and can handle the meltdowns with calm too.