Hi 
yes as others say, a seat with a harness until he’s four would be ideal. Do you know what centile he’s on/if he’s big for his age? As if he is it can be more worthwhile to invest in a 25kg seat. Most harnessed seats go up to 18kg though there are some on the market that harness up to 25kg (average 7 year old).
Yes it would be possible for him to go forward facing, though rear facing is safer. A spin seat would be a good one as he can try rear facing and if he doesn’t like it can go forward facing (if it isn’t a RF only one!). These tend to be isofix only though.
Isofix can make the seat easier to install, as they’re two metal prongs that click into two bars under the seat of the car. Most cars after 2004 have isofix and after 2013 for top tether, which helps to stabilise the seat and is useful in forward facing only seats. In harness seats they’re used to keep the seat in the car and some high back booster have options but that’s to keep the seat stable in the car and so DC doesn’t push it over the seatbelt and stops it being a projectile in a crash as it can be a bit of a faff to strap it back in! Generally it’s safer to buy one car seat for up to ~4 years old then a separate high backed booster. I’d stick to the well known brands, Joie, Nuna, Cybex, Britax and Maxi Cosi as non-branded ones (Nania/Team Tex) are cheaply made and don’t do as well in crash tests as they’re only crash tested to the legal minimum whereas the other aforementioned brands test higher
Good forward facing seats
Britax Duo Plus, is both seatbelt and isofix
Britax Trifix 2 - isofix and top tether only
Maxi Cosi Tobi - seatbelt only
Britax king - seatbelt only
Spin car seats
Britax Dualfix 2R - I rather like this one as it’s reasonably priced and scores well on safety
Joie Spin 360 - also reasonably priced and very popular, though in safety doesnt do too well on side impact
Joie i-spin 360 - like the regular 360 but i-size. Does better on safety and has a different rotation mechanism which IMO is easier. Though as it’s i-size it is somewhat restricting in high, as once DC reaches 105 centimetre they can no longer use it. Eg if long legged and short torso at 105cm while physically they may be able to fit in the shell as they’re over the height limit it can no longer be used!
Britax Dualfix i-size/i-size M. The m edition is cheaper as it doesn’t have a newborn insert so not suitable from birth, though obviously not relevant unless you’re planning DC2 and don’t want to use an infant carrier.
Cybex Sirona - there are many variants of this. There is the S which remains attached to the base, the Sirona M isn’t a spin seat though is modular (uses same base for two stages of car seat), then there’s the Z which is a modular seat and takes the infant carrier too on the base (Cloud Z). This could be useful if switching car seat between cars if you have one base in each car you only have to take the seat unit off instead of the whole seat out the car. Though combined is very expensive
Maxi Cosi have a 360 family too, the 360 Pearl which is rather new to the market. Like the Sirona Z can be detached from the base and is a modular seat. Though unless you plan to have DC2 when DC1 is around 3-4 it probably wouldn’t be worth it.
Maxi Cosi also have more seats that swivel—the axissfix which can only rear face to 87cm, the Axiss which is seatbelt installed and forward facing only. There’s also the Mica which can both rear and forward face up to 105cm as it’s an i-size seat.
GB Vaya which is very similar to Cybex Sirona S.
Nuna Rebl is also rather popular option
Britax are also bringing out a new car seat in May that they’ve said is “the biggest car seat innovation of 2021” so might be worth waiting for as they do rather premium seats.
Phew, I think I’ve covered everything 😅