is the central cabinet fixed and is the original fireplace opening closed off? then i agree with Dianne and would leave out the central cabinet and have wide horizontal shelves for toys and books across the front of the old fireplace but only up to about 1100 high, Ikea makes a range of flat pac book shelves but always securely fixed to the wall and use the blank wall above the shelves for artwork ..something like this

Like okl image.
It will be cheaper to replace the unit than to do a lot of fiddly work like creating base units to get base of all units to skirting height and even at top and run a skirting around, make fill in panels, change or remove doors on middle cabinet as don’t match etc...

thank you for your responses Irene, Louise and Kate. I do like the image you sent but my concern is we don't know what is behind the middle cabinet and don't particularly want to deal with potentially having to patch a hole in the wall etc And am trying to keep further costs to a minimum.

am leaning towards adding bridging shelves (possibly oak) to the center unit for a bit of a contrast and it goes with the furniture he has in his room. thoughts about that?

any custom built shelving will be more expensive than a simple flatpac bookshelf but i would want to inspect the space behind the central cabinet to make sure it was properly sealed and make sure that any cabinets or shelves are screwed into the walls for safety

Putting a back on a set of shelves would be cheaper than your infill proposals

The problem is you've got freestanding furniture where a built-in solution would have been better. It's always going to look like free standing furniture. To improve the look the cabinet in front of the fireplace needs to go entirely. You can then build shelves onto the chimney brest to meet the front of the wardrobes but those shelves aren't going to be anything more than picture rails. Doesn't matter what wood you use because you'll paint it the same colour as the wardrobes.

It's an awkward solution. It would have been more cost effective to have the wardrobes built in either side of the chimney brest, full height to meet the ceiling with mouldings that matched the rest of the room. You could have then decorated the chimney with a large mirror and it would have looked 100% better.

As it is I would just have the two wardrobes there, get rid of the third and leave it at that.