At 16 months it's likely that he is too big now for the 3-point harness which is included with infant seats. These are sufficient to hold a passive child in the seat in a collision but not to contain a wriggly escape artist. A determined child can easily push an arm through the straps (this is still possible with a 5ph but less easy) or a slight one can wriggle enough that their shoulders pop out of the top - this isn't as possible with a 5ph.
I'm probably too late to help you with this but will post anyway - Halfords sell a device called Five Point Plus which you can add to any car seat to make it less escapable. I'm surprised he hasn't sold you that TBH. In the absence of that option, I'd buy a pair of reins or other removable harness and put it on under the car seat straps with the car seat straps threaded through it until you get home, to buy you thinking time re upgrading the seat, which I do agree is urgent. (I wouldn't use the reins trick permanently).
Longer term - yes forward facing is safe and legal at this age/weight, but it's not the safest. I certainly don't agree that any cheap forward facing seat is fine, BTW, because some of the cheaper seats are horrendous, but that said - they are better than a totally unrestrained child.
If it's possible for you to borrow a seat with a five point harness - whether rear or forward facing - I would do that and try it out for a few trips to see if he can escape from it. Lots of children can just by sucking in their tummies and if he's 16 months old, he's not likely to respond to any scare or reason based tactics to stop him doing it. Rear facing might be safer, but if he's rear facing and you can't easily see if he's fully restrained then that spells disaster - I mean, realistically, he could have been slipping his arms out of his current seat straps for weeks and you might not have noticed if he put them back in again.
If he's escaping the 5ph as well then you can either try one of the two options which make this more difficult again - the 5 point plus accessory as mentioned or there is a "belt collector" made by BeSafe which holds the shoulder straps together. There's another product called Houdini Stop but this isn't recommended because it's untested and might place undue force on the straps in a collision (though - again, maybe better than an unrestrained child).
Or the other option would be to look at an impact shield based seat, which doesn't use a harness at all so is less escapable from that point of view - but a skinny child can, apparently, push slowly at the shield itself and then squeeze/climb out the top of it. You can't get these in rear facing.
There is one seat which is a 5ph when rear facing and an impact shield when forward facing which might be worth looking at?
Hope you find something that works, anyway.
