I just found baby boy clothes on sale at Target today. I bought shirts for $1.40 and .98 cents each and shorts for .98 cents each. I stock up whenever I find a good deal like that. Old Navy has cute clothes. Also The Children's Place has really cute clothes and the one by me puts their whole store on sale for .99 cents each sometimes. All the other times their items are usually at least 50 percent off.
Babies grow at different speeds. I would get lots of footie pajamas in 0-3 and 3-6 months. 6-9 months would likely be summer clothes and maybe 12 months, too.
My baby was larger 9 lb, 2 oz. He was in 3-6 months by 6 weeks old. He was in 6-9 months by 3 months. 12 months by 6 months. 18 monthes by 8 months. He is in the 97% for both height and weight.
Oldest was a preemie. She was wearing 3-6 months on her first birthday. It varies a lot.