Getting stuck on one or two things is quite normal for little guys of this age. It’s always good to offer alternative ideas to expand his horizons as well. Next time when you’re reading a story- point out something that would make a good picture. Encourage him to paint it or extend the scene by adding his own parts.

You could use a playdough ‘S’ and turn it into a snake and show him how letters can morph into other things.
It’s important to embrace the letter obsession too every now and again and not always try and move him on to something else.

I think the language you use will be important to not complaining about ‘alphabet or letters again!’ but instead using everyday opportunities , as they arise, to inspire his creativity. ” look at that b i g tree! We should draw one like it at home” Or remember a good experience and ask him to draw it for you. You can do it together. Scaffolding his ideas by helping him go a bit further than he can already.

To keep his imagination and creativity going or kick start it again; read lots of books, have them about and tell stories. Limit TV and passive activities especially electronics that require him to just push buttons. Encourage play with Have plenty of paper on hand for them to doodle whenever they want. Pin up their work and praise how hard they worked on it rather than what they produced. “Wow you worked hard using the blue today” ” Can you tell me about this?”

Hope some of these ideas work for you. With time he’ll be onto other things……. mine have a superhero obsession right now.