What’s the best way to determine the right cricket bat for my son. He is 16 and plays junior reps senior club cricket.
He’s around 165cm and probably due for a growth spurt in the next 18 months.
Also, a friend mentioned due to his height should we look to get a bat with a sweet spot towards the lower end of the bat. Is that correct?

Hi Damian, bat selection can be quite personal and to suggest there is one answer would be dangerous.
Pick Up or back lift = the player needs to be able to pick the bat up (back lift) and feel in control. Some bats 'pick up' better than other bats even though they are the same weight. Try a number of bats and get the feel of this. If the player can consciously feel the bat weight when picking it up, it is a sign the bat is either too heavy or not right for them.
Weight = the weight of the bat should allow the player to be successful in the cricket they are playing / aspiring to play. A lighter bat will be easier to use and manipulate, (particular reference to fast and short pitched bowling), a heavier bat if swung with speed will produce more power. My experience is too many younger players go for bat weight/mass at the expense of bat control and pay the price. You won't know the full weight you can handle until you are a mature adult.
Handle length = personal choice and depends on the grip(s) the player employs. Gilchrist held right up the top, while Beth Mooney holds right down the bottom. Both incredible cricketers. A shorter handle is more often chosen these days, but that doesn't make it right for everyone.
Durability = bats are expensive, so I can't advise you on the quality or cost. I would suggest a senior size bat to guarantee the longest life possible for use. If your son is 165 and heading up it would appear a senior size bat would be appropriate. Try and see how they feel.
Sweet spot = the lower sweet spot is generally associated with slower and lower bouncing turf wickets. They are more often seen overseas than in Australia. Having said that, these things are personal. Personally, I wouldn't go for a bat with a lower sweet spot in Australia for what it's worth.
Have fun, a new bat is an awesome time for a young player. Enjoy.