If he's 18mos and still rear facing, leave him there! A rear facing car seat is outgrown when one of these things happens... the child goes over the rear facing weight limit or there is less than one inch of hard shell above the childs head.
If he's in an infant seat, purchase a CONVERTIBLE car seat and keep him rear facing. Here are some good ones.
Cosco Scenera is $50, will rear face to 35lbs (new ones to 40lbs), forward face to 40lbs, and is a good bare bones seat. Its a good safe seat for a low price. Its not padded much but its still very safe and should last most kids to age 3 when another higher harness weight seat should be purchased.
Evenflo Triumph ADVANCE is $130 and will rear face to 35lbs and forward face to 50lbs. Great seat, no thread harness adjustments! Will fits most kids to age 5 forward facing, its a really nice padded and lots of features seat for a low price.
First Years TrueFit is $170-$230 will rear face to 35lbs and forward face to 65lbs. Nice tall seat that fits older kids well, most kdis can use to age 6/7.
Graco MyRide 65 is $150, will rear face to 40lbs, forward to 65lbs. EXCELLENT seat for extended rear facing and is a big comfy padded seat. Fits most kids to 4 rear facing and 5/6 forward facing!
Sunshine Kids Radian is $200-$300 will rear face to 40-45lbs, forward face to 65-80lbs (depending on model) and is a narrow seat, fits three across and into smaller cars easily. NICE seats.
Britax Marathon or Boulevard is $270-$300 will rear face to 35lbs (new ones will to 40lbs) and forward face to 65 (new ones will to 70lbs) I love our Britax seats ,the yare well made, easy to use and install too.
If you are insisting on moving him to a forward facing seat, the Graco Nautilus is a great seat. It will harness to 65lbs, booster to 100lbs. Its $150 and will be the last seat he will need.
The Evenflo Maestro is $70, will harness to 50lbs, and then becomes a booster seat. It is a great seat for the price and makes a good booster seat.
The Evenflo Generations 65 is $99 and will harness to 65lbs and then booster. Again a great seat for the money.
Britax Frontier will harness to 85lbs, but yoru child must be 2yrs old before using it. Its $280 and a great seat, will booster to 120lbs.
Consider keeping him rear facing longer, google EXTENDED REAR FACING and JOELS JOURNEY for more information. IF he's in a convertible check to see if he's outgrown it by reading what I posted above. If he's still within the limits of the rear facing seat, leave him there, or purchase a convertible to keep him rear facing longer. If you are insistent that he go forward facing, look for one of hte four I recommended, they will last the longest and are the best.