What military uniform? Help, please

[Time Runner]

Yes, that's almost certainly a capbadge on the side. As can be seen from the second photograph, he doesn't have a capbadge on the front of his hat, which reinforces the idea that it's on the side.

Right, thanks, Andy.

It will not be a badge but a cloth flash or patch which will designate his unit.

Understood. Thanks, alan. Trying to get my head around the terminology here: when you say "unit", would that be a regiment or something smaller. And also, if it's a "cloth flash or patch" (rather than a badge), would that help to narrow down the army he was part of?

[Andy J2022]

While I don't disagree with alan o that it could be a cloth badge, I wouldn't completely rule out a blackened metal capbadge. The Australians certainly wear metal badges that way. As for the British Army, if it is cloth I would expect it to be a regimental (yes, that's the same as 'unit' in this context) badge rather than a formation flash since he's already wearing the formation flash on his shoulder.
Incidentally, one of the reasons suggested for why cloth badges were preferred for jungle operations is that they didn't glint in the sunlight as a metal badge might. In jungle operations two opposing forces generally get much closer to each other compared to terrain like NorthWest Europe or North Africa where contact engagements tended to be at hundreds of metres, at least initially. Whereas movement and sounds in the jungle can have natural (animal) causes, a glint of sunlight on metal can only mean human activity.

[alan o]

Usually it was a regimental or even a battalion pattern.  There was even a difference between individual battalions of the same regiment. 

It is far too large to be a cap badge.

It will not be a badge but a cloth flash or patch which will designate his unit.

Understood. Thanks, alan. Trying to get my head around the terminology here: when you say "unit", would that be a regiment or something smaller. And also, if it's a "cloth flash or patch" (rather than a badge), would that help to narrow down the army he was part of?
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[alan o]

It is not the same patch but this is a  good clear picture of another unit wearing them on the side: 

https://www.iwm.org.uk/collections/item/object/205205185

[Nanna52]

I am by no means an expert, but Australians had the Rising Sun badge on their hats.  Battalion colours were on their sleeve.

[alan o]

I am by no means an expert, but Australians had the Rising Sun badge on their hats.  Battalion colours were on their sleeve.

Quite correct but this is the British army rather than the Australians.  Totally different.

[Time Runner]

Thanks for your replies, Andy, alan and Nanna, all very interesting. (The jungle warfare info was fascinating, Andy.)

alan, you say that:

[...] this is the British army [...].

Now, that's a key piece of info, right there! Up until this point in the thread it's not been clear to me if there was a view as to which allied nation this guy was fighting for.

What makes you certain that he's British army, alan?

And more generally, can any of you recommend any further sources that I might use to try and identify him?

[Andy J2022]

[...] this is the British army [...].

Now, that's a key piece of info, right there! Up until this point in the thread it's not been clear to me if there was a view as to which allied nation this guy was fighting for.

What makes you certain that he's British army, alan?

And more generally, can any of you recommend any further sources that I might use to try and identify him?

I assume alan o is basing his statement on tonepad's discovery (post 3) that the shoulder flash was that of the 20th Indian Infantry Division. This Division was composed of British, Indian and Gurkha troops. I doubt if this man was serving with the Indian Army - it is possible he was attached but he's not an NCO so I suspect not - therefore he is most probably a member of one of the British Army units.
You can find the order of battle for the Division for the period 1944-45 here. The division was first formed in 1942, was transferred to 4th Corps in July 1943 and later took part in the Battle of Imphal and the Burma Campaign more generally. He would have been part of the 'Forgotten Army' (General Slim's Fourteenth Army).

Addendum. The orbat I linked to is incomplete as it doesn't shown shown most of the supporting troops such as RAOC and RASC, etc who would have been responsible for the logistic support of the fighting troops (which are the ones listed).

[Time Runner]

That's great, Andy, thank you for the leads. There's a lot to dig into (almost all of it new to me), so I'm going to need some time to do some serious research. Thanks again for all your help. It's much appreciated!