It is much harder than one would think. The best thing to do would be to have his parents take some measurements and buy a current pattern that reflects those measurements. I've sewn for 50 years and I wouldn't attempt to grade patterns (changing them from one size to another) For instance, his chest measurement might be two inches bigger than a 4 year old, but certainly his neck *won't be,* and those two things have to be combined. Little kids' bodies change, too. A toddler pattern (can go up to 4T) is shorter, and sized to accommodate a diaper. Children's sized patterns are taller, maybe a touch slimmer, and labeled without the T.

They could send you clothing that fits, and you can try to adjust the patterns to match the clothing.

Why not just buy a new pattern? a See and sew pattern is $2.99, but even a full priced pattern would more than save the cost of wasted fabric for something that doesn't fit. Patterns are almost always in sale someplace.

I would make a pattern from scratch rather than grade one to fit, 90% of the time.

You said 'old' McCalls patterns...I agree with c t's advice above. Patterns evolve with the body changes in generations. If, for instance, you would wear a size 12 dress today, an old pattern might be labeled 'size 12' but would be too small and proportioned wrong for today's size 12 person. I'm fairly sure that children's patterns have evolved in size over the years. You might resize the old patterns with the child's body measurements to help you. Fabric is too expensive today to make mistakes when cutting. JMO.

Thank for your ideas and feedback.
One of the reasons I looked at old patterns was for the traditional and cute designs.
Today’s boys clothing is just so baggy and sporty and not as formal smart and cute as it use to be.
It makes complete sense that times and body shapes change and proportions differ with age.
I’ve tried making patterns from scratch before by basing on existing garments with differing success.
Can anyone recommend a pattern supplier for smart formal clothing.
I’ve attached a photo of one attempt last Christmas

https://voguepatterns.mccall.com/patterns/girlsboys

Often, it's the fabric that makes the difference between casual and formal. I would not call the outfit pictured a formal one.

I would search for 'classic' or 'heirloom' sewing patterns, such as Martha Pullen or Children's Corner. Here's a good place to start: http://www.heirloomsewingforchildren.com/pattern_index.htm

And, while not sewing pattern related, this blog "What Kate's Kids Wore" might be helpful in identifying the styles you are seeking, since she tends to dress her children in a classic manner. If you open a post, the writer goes into detail about each child's outfit: https://whatkateskidswore.com/2018/11/prince-louis-princess-charlotte-prince-george-are-darling-in-prince-charles-birthday-pictures/#more-3730